Make nova-compute-{kvm,qemu} packages overridable

To keep the "we define platform choices in the attributes" spirit, the
nova-compute "support packages" for ubuntu should be overridable, too.

This change makes them behave just like  to the
"#{db_type}_python_packages" in openstack-compute::nova-common.

Change-Id: Id38169121e52e0dc8be808c8822db55069e65e48
Merge "Make nova-compute-{kvm,qemu} packages overridable"