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])
Allow virtualenv definitions to share the same requirements file
Merge pull request #6 from zoni/sharedrequirements

Allow virtualenv definitions to share the same requirements file