gentoo-python: Remove Python 2

Portage runs fine on Python 3, and there's no need to have multiple
versions of Python installed.  Removing it here allows us to trim down
to smaller images, because Python's pretty heavy:

  $ docker run -i -t --rm wking/gentoo-portage:20140717 /bin/bash
  d1cdbe4c737a / # du -hs /usr/lib64/python2.7/
  66M     /usr/lib64/python2.7/

My current trimmed ngircd is only image is 341 MB:

  $ docker images wking/ngircd-trimmed | grep 20140717
  wking/ngircd-trimmed   20140717   98daec1a09c5   16 hours ago   340.9 MB

so about 20% of the trimmed image is a useless old Python version.  We
could shrink the trimmed image even more by removing Portage and
Python 3, but that requires some external-to-Portage hackery.  This is
easy to do, and folks who *do* need Python 2 can easily adjust their
PYTHON_TARGETS, PYTHON_SINGLE_TARGET, eselected Python, and re-emerge.

We only need to emerge @system, because @world is still empty at this
point.
buildbot/Dockerfile.template: Adjust Python 2 dependency

Now that the ancestor gentoo-python image is removing Python 2, we
need to get it back for Buildbot [1,2].  We need to keep the
USE_PYTHON setting (with a tweaked value) because buildbot hasn't
transitioned to the *-r1 Python eclasses yet:

  $ grep 'inherit.*distutils' /usr/portage/dev-util/buildbot*/*ebuild
  /usr/portage/dev-util/buildbot-slave/buildbot-slave-0.8.7_p1.ebuild:inherit distutils readme.gentoo user
  /usr/portage/dev-util/buildbot-slave/buildbot-slave-0.8.8-r1.ebuild:inherit distutils readme.gentoo systemd user
  /usr/portage/dev-util/buildbot-slave/buildbot-slave-0.8.8.ebuild:inherit distutils readme.gentoo user
  /usr/portage/dev-util/buildbot-slave/buildbot-slave-0.8.9.ebuild:inherit distutils readme.gentoo systemd user
  /usr/portage/dev-util/buildbot-slave/buildbot-slave-9999.ebuild:inherit distutils readme.gentoo user
  /usr/portage/dev-util/buildbot/buildbot-0.8.7_p1-r1.ebuild:inherit distutils readme.gentoo user
  /usr/portage/dev-util/buildbot/buildbot-0.8.8-r2.ebuild:inherit distutils readme.gentoo systemd user
  /usr/portage/dev-util/buildbot/buildbot-0.8.9.ebuild:inherit distutils readme.gentoo systemd user
  /usr/portage/dev-util/buildbot/buildbot-9999.ebuild:inherit distutils readme.gentoo user systemd

Also, instead of hardcoding the packages effected by the sqlite USE
flag change, just use --newuse on @system and @world and let Portage
figure out what needs to be rebuilt.

[1]: Buildbot will transition after Twisted
     (http://trac.buildbot.net/wiki/CompatibilityPolicy#Python3)
[2]: Twisted is 60% of the way there (94/157 Python 3 tickets,
     https://twistedmatrix.com/trac/milestone/Python-3.x)
docker-registry/Dockerfile.template: Adjust Python 2 dependency

Now that the ancestor gentoo-python image is removing Python 2, we
need to get it back for docker-registry.  We need to keep the
USE_PYTHON setting (with a tweaked value) because Gunicorn and
Python-RSA haven't transitioned to the *-r1 Python eclasses yet:

  docker run --rm wking/docker-registry /bin/bash -c 'grep inherit /var/db/pkg/  */*/*ebuild' | grep distutils | grep -v -- '-r1'
  /var/db/pkg/dev-python/rsa-3.1.1/rsa-3.1.1.ebuild:inherit distutils
  /var/db/pkg/www-servers/gunicorn-0.17.4/gunicorn-0.17.4.ebuild:inherit distutils eutils
gentoo-layman/Dockerfile.template: Work around the lack of Python 2

The existing stable version of Layman needs Python 2, but the ~amd64
version is compatible with Python 3:

  $ cd /usr/portage/app-portage/layman
  $ grep 'PYTHON_COMPAT\|KEYWORDS' *ebuild
  layman-2.0.0-r1.ebuild:PYTHON_COMPAT=( python{2_6,2_7} pypy2_0 )
  layman-2.0.0-r1.ebuild:KEYWORDS="... amd64 ..."
  layman-2.0.0-r3.ebuild:PYTHON_COMPAT=( python{2_6,2_7} pypy pypy2_0 )
  layman-2.0.0-r3.ebuild:KEYWORDS="... amd64 ..."
  layman-2.1.0-r2.ebuild:PYTHON_COMPAT=( python{2_7,3_3,3_4} pypy )
  layman-2.1.0-r2.ebuild:KEYWORDS="... ~amd64 ..."
  layman-9999.ebuild:PYTHON_COMPAT=( python{2_7,3_3,3_4} pypy )
  layman-9999.ebuild:KEYWORDS=""

Layman also needs Git, and with the 'python' USE flag enabled, Git
needs Python 2:

  $ cd /usr/portage/dev-vcs/git
  $ grep PYTHON_COMPAT *ebuild
  git-1.8.3.2-r1.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-1.8.4.5.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-1.8.5.5.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-1.9.3.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-2.0.1.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-2.0.2.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-2.0.4.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-9999-r1.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-9999-r2.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-9999-r3.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-9999.ebuild:PYTHON_COMPAT=( python2_{6,7} )

The only interesting Git features enabled by the 'python' USE flag are
the Bazaar and Mercurial remote helpers, and even they were removed in
Git 2.0.0 [1,2].

[1]: https://git.kernel.org/cgit/git/git.git/commit/?id=b2c851a8e67da752d8a5dbde5a9dae6e3428a4c9
[2]: https://git.kernel.org/cgit/git/git.git/commit/?id=896ba14d653c43bd7a43b6b291e941ab60855c04
Merge branch 'python'

* python:
  gentoo-layman/Dockerfile.template: Work around the lack of Python 2
  docker-registry/Dockerfile.template: Adjust Python 2 dependency
  buildbot/Dockerfile.template: Adjust Python 2 dependency
  gentoo-python: Remove Python 2
docker-registry/Dockerfile.template: Add flask-cors dependency

This landed in docker-registry's be21b160 (Fix issue #333 - Added CORS
support to 'get images' and 'search' methods in registry REST api,
2014-05-01), which landed in 0.6.9 (2014-05-08).  I need to use Layman
and my overlay until the flask-cors ebuild lands in the main Portage
tree [1].

[1]: https://bugs.gentoo.org/show_bug.cgi?id=517478
build.sh: Bump DATE to 20140904

This pulls in a fix for Layman on Python 3 [1,2].

[1]: https://bugs.gentoo.org/show_bug.cgi?id=521724
     Summary: app-portage/layman-2.1.0-r2 - ImportError: cannot import
       name 'MakeConf'
     Devan Franchini (twitch153) cut layman-2.1.0-r3 with a fix [2].
[2]: http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/app-portage/layman/layman-2.1.0-r3.ebuild?revision=1.1&view=markup
     Summary: Adds patch to fix improper import for ConfigHandler, bug #521724
     Date: 2014-09-02 22:06:59Z