Clean up test_extensions.py imports

Remove unnessesary imports
Import only modules

Change-Id: I4988714e3ae31ce77f3a2c1e27e81dd6258c7546
Use install_venv_common.py from oslo.

This syncs install_venv_common.py from oslo and reworks the
tools/install_venv.py script to use the new library.

Change-Id: I46bbf7d656e6cb9660e37af2d3ce96150e18e7fe
Merge "Clean up test_extensions.py imports"
Merge "Use install_venv_common.py from oslo."