Allow venv access to system packages

- Adjust the virtualenv creation so that the created venv
  has access to the system (parent) packages so that when
  clients are installed anvil will have access to them to
  do post-start actions.

Fixes bug 1294653

Change-Id: I9409feade0b28b3f06dc872f673f4745e7cb3d38
Merge "Allow venv access to system packages"