[nova][ptg] Resource provider delete at service delete

ML thread: http://lists.openstack.org/pipermail/openstack-discuss/2019-June/007135.html Agreements in the room: * Check ongoing migration and reject the delete if migration with this compute having the source node exists. Let operator confirm the migrations * Cascade delete providers and allocations in placement. * in case of evacuated instances this is the right thing to do * in any other dangling allocation case nova has the final thrut so nova has the authority to delete them. * Document possible ways to reconcile Placement with Nova using heal_allocations and eventually the audit command once it's merged. Cheers, gibi