Stabilise Bifrost CI job

The Bifrost CI job seems quite unstable, often failing while attempting
to reach the public epel package repositories. We shouldn't need to
install any packages when deploying the container - they should all have
been installed in the image already.

This change avoids running the scripts/env-setup.sh script, which is run
when the container image is built.

It also removes sourcing of /opt/stack/ansible/hacking/env-setup, which
is now just a stub.

Change-Id: I1786e5337a397cb7b427d6b87c21eaee600af170