General-host-aggregates part 1.
General-host-aggregates part 1.

Partially implements blueprint general-host-aggregates: Xen decoupling

* keep rpcapi to compute-controller for Hypervisor-Pools
  *  Add compute_rpcapi.add_aggregate_host to libvirt
* Remove aggregate states
* Remove One aggregate per host restriction
* Keep Xen support for both hypervisor pools and aggregates

Change-Id: I6ac0f8b6fc31dff589363331e99e0755301f2172

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects