Use "user" instead of "runas" argument when calling pip.install from virtualenv.managed state

This commit changes the virtualenv.managed state to use "user" instead of "runas" when it runs pip.install to satisfy a requirements file.

Additionally, it deprecates the "runas" argument in the virtualenv.create function. This is part of a greater movement towards replacing "runas" with "user" where possible.

Fixes #8190.