Run bifrost inside python virtual environment

Bifrost can now run within a python venv:
https://docs.openstack.org/bifrost/rocky/install/virtualenv.html

After this patch, the bifrost provisioner will use python
installed inside the virtual environment and will be
transparent of the system python installation.

Change-Id: I590a369ed1f9631a167d4a9084c040e7561b9c53
7 files changed