Collect libvirt metrics only for libvirt compute driver

Some drivers like ironic, vmware and others will not require libvirt is
running on local compute.
This patch updates prometheus.yml metadata to configure libivirt metrics
collection only when compute_driver == libvirt.LibvirtDriver

Change-Id: I3a10e985afbbea826043d9ae341f298fe8402136
Related-Prod: PROD-25643
(cherry picked from commit 15edf39469c3341a40531503fd7f7a659d77d401)
1 file changed