bump(dev-lang/python): Update to python 2.7, ebuilds from chromiumos.

They have some patches to fix cross-compiling in this build environment.

Start off simply by adding 2.7 as a new parallel install target, will
switch to 2.7 as the primary version in a later step.
fix(profiles): Switch to python2.7 as the one and only version.
fix(coreos-base/coreos-dev): Replace dev-python/argparse with virtual.

argparse is bundled into python-2.7 so dev-python/argparse cannot be
installed. Instead the virtual package should be used to work with any
python version combination.
fix(profiles): Enable dev-lang/yasm-1.2.0-r1

This version fixes XML errors and python eclass usage.
Merge branch 'master' of ssh://github.com/coreos/coreos-overlay