Add inline docs for common.yaml

The common.yaml file lacks inline documentation, which makes it harder
for users to understand.  This patch adds inline documentation to
make the file more self-documenting.

Change-Id: Iba98f2a829ff9b5b8ae6d2242544f0507866567c
Partial-Bug: #1270381
Enable nova to do live migration

Set the nova::compute::libvirt::migration_support param
to true by default. It was previously no present.

Change-Id: I3657969e0a921b406e14b44aae15912bd80103e6
Merge branch 'master' of https://github.com/stackforge/puppet_openstack_builder into coi-development