Add get_by_service() method to ComputeNodeList object

This allows us to get the ComputeNode objects related to a single
Service, in the case of the many-to-one situations of baremetal
and vmwareapi. In order to avoid passing the whole Service over
RPC, this adds a non-remotable API in front of the RPC one that
takes a full service, but ends up only passing the ID.

Related to blueprint compute-manager-objects-juno

Change-Id: I995fa6948bda64d2f5d60f0c81270321133bbfa8
Merge "Add get_by_service() method to ComputeNodeList object"