Enhance virtualenv module to install distribute/pip

Unlike virtualenv, pyvenv in python 3.3 (and virtualenv in future versions) doesn't install distribute and pip by default. It makes it impossible to use managed virtualenv state because it tries to find pip in virtual environment immediately after its creation.

To solve this problem I have added support for installing distribute with optional pip after pyvenv/virtualenv creates empty virtual environment. This way, when called with `pip=True` parameter, the environment will be immediately ready for installing requirements.txt using pip.

Prerequisite for #5494. Fixes #6029.