Remove nova-compute from boot-stack dependencies.

As things have been factored out, boot-stack no longer implies
nova-compute on the same host. This specifically addresses the case
where we build the overcloud-control image and it fails to restart
nova-compute during post-configure because nova-compute defaults to
libvirt, but libvirt is only installed by the nova-kvm element.

Change-Id: I3cba8a48c64d3919898d4ea47a6cd7808327cd9c