Stop using distribute in install_venv_common

Upstream distribute is dead and has been merged back into setuptools.
Additionally, enhancements have been made to both setuptools and pip
that mean we no longer have to carry hacks around versions.

Also, we don't need to install greenlet here. Order is respected in
requirements.txt.

Also, let's kill the pip_requires verbage.

Change-Id: Ia15c66d6a03522e804ff96fd6f8d81db00467a6e
Merge "Stop using distribute in install_venv_common"