Expose libvirt_cpu_mode parameter

Some users who are deploying OpenStack on nested hypervisors
(common in test lab environments) may wish to change the libvirt
CPU mode in nova.conf to expose CPU settings.  This setting is
now parameterized in the nova::compute::libvirt class.  This patch
exposes it via common.yaml.

Change-Id: I3d96c754e67343988831ddf949221ffd3a301281
Implements: blueprint expose-hw-virtualization
Merge "Expose libvirt_cpu_mode parameter"