Remove all tasks and variables related to toggling between installation
of horizon inside or outside of a Python virtual environment.
Installing within a venv is now the only supported deployment.
Additionally, a few changes have been made to make the creation of the
venv more resistant to interruptions during a run of the role.
* unarchiving a pre-built venv will now also occur when the venv
directory is created, not only after being downloaded
* virtualenv-tools is run against both pre-built and non pre-built venvs
to account for interruptions during or prior to unarchiving
Change-Id: I8f5d33b90cc2c41beca2067f0f99d8c1283021e4
Implements: blueprint only-install-venvs
4559edf920