Make sure openstack services are dependent on openstack-core

Make sure openstack services are dependent on openstack-core

Previously ceilometer-notification, aodh-listener and sahara-engine
didn't have constraints that would anchor them under openstack-core
dummy resource. Such constraints are added now. (sahara-engine starting
after sahara-api, aodh-listener after aodh-evaluator, and
ceilometer-notification after openstack-core.) Openstack-core ->
heat-api constraint has been removed because heat-api depends on
ceilometer-notification, so there's a transitive dependency on
openstack-core already.

Change-Id: Ided7321ebbf2c3556726343b4bb466fd8759b43a
Closes-Bug: #1569444