Description of problem:
This Bugzilla tacks the downstream feature backport of
https://blueprints.launchpad.net/nova/+spec/weigh-host-by-hypervisor-version

The HypervisorVersionWeigher enables the nova scheduler to express a preference for a given host based on the version fo the hypervisor installed.

This is useful for upgrades, hardware replacement and mixed rhel testing.
The backport of this OSP 19 feature is done to enable mixed rhel testing
during FFU to osp 17.1 form 16.2

This bz tracks the initial backport to OSP 18.0 and will be cloned to track
the final backport to 17.1.