installer images created via libvirt or container

When using rpm-os-toolbox to create installer images (PXE and isos),
lorax and friends really want to run in the native operating system. So,
for example, creating a centos installer image on a Fedora system will
fail.  We now by default use a container to create the image.  You can also
pass --virt to use imagefactory and indirection.

* installer.py
    - createContainer() does the work for the container bits
    - createContainer requires docker_os_name which defines the container
      base OS.  Like fedora:rawhide or centos:latest
    - create uses indirection
    - subjected entire file to pep8 and flake corrections

* imagefactory.py
    - small changes to allow installer.py to reuse some of its classes

* taskbase.py
    - added docker_os_name to ATTRS
    - added parsing of util_uuid which allows to re-use and existing
      imagefactory utility image to cut down on processing time

* lorax-http-repo.tmpl
    - New file for using ostree httpd sync'ing

* lorax-indirection-repo.tmpl
    - New file for using imagefactory indirection to build installer images

* Makefile-toolbox.am
    - added installation of the lorax-http-repo.tmpl
    - added installation of the lorax-indirection-repo.tmpl

* rpm-ostree-toolbox.spec.in
    - Add requires for docker and IndirectionCloud
installer: Use a with context for open

Just a cleanup.
installer: Honor $http_proxy for lorax

This code existed before.  It still works in the Docker world because
we use --net=host.
installer: Only clean yum context when done

Otherwise we download 30MB repo metadata each time.
installer: Properly quote values when writing lorax shell script

We're generating a shell script, so we need to quote the arguments.
Don't concatenate "-s repo" as one string either.

This fixes using $basearch in a repository URL, among other things.
installer: Print lorax command line

For debugging.
installer: Use $baseos/rpmostree-toolbox-lorax for Docker image name

Overwriting the base is bad!  We want a different image.  I noticed
this because I hit the 127 parents limitation.
installer: Support lorax_exclude_packages

Currently in rawhide (F22, as of 20141206), lorax grabs all product
packages.  Thus, one has to exclude the others on the command line.

See:
https://lists.fedorahosted.org/pipermail/anaconda-patches/2014-December/014956.html
installer: Use bind mounts, allow caching Docker image, etc

This patch ended up being a big ball of stuff together.

 - Don't use docker cp; instead, use bind mounts.  This is more
   efficient, and allows us to also use --rm, to prevent accumulating
   container images.  We also use --workdir to more easily capture
   the lorax logs.
 - Add --skip-subtask docker-lorax to reuse the cached one.  For
   local developer use.
 - Make outputdir required, for the same reason it is with imagefactory.
   In the future, we should investigate having better defaults.