libvirt: add version cap tied to gate CI testing

Limit the libvirt features that Nova will use in a default
configuration to only those which are tested in the CI gate.

The end user can override the limit if they wish to opt-in
to use of untested features via the 'version_cap' setting
in the 'libvirt' group.

Change-Id: Ib91164cc572b1dbde2a31beee5751c52b0445d3e
Merge "libvirt: add version cap tied to gate CI testing"