Resolve a cyclic dependency

There was a dependency between python::virtualenv and
python::requirements. Basically, the virtualenv wanted to initialize
requirements, but requirements depended on the virtualenv being created
already, resulting in:

err: Could not apply complete catalog: Found 1 dependency cycle:
(Exec[python_requirements_check_/usr/share/err/repo/requirements.txt] =>
Python::Requirements[/usr/share/err/repo/requirements.txt] =>
Exec[python_virtualenv_/usr/share/err/python3] =>
Python::Requirements[/usr/share/err/repo/requirements.txt] =>
Exec[python_requirements_check_/usr/share/err/repo/requirements.txt])
Make pip install distribute in a virtualenv optional

pip install distribute currentlyfails on Python 3, this way people can
skip installing it. It's included by default to remain backward-
compatible
Merge branch 'distribute'
Allow virtualenv definitions to share the same requirements file
Allow virtualenv definitions to share the same requirements file
Merge branch 'sharedrequirements'