Add support for batched upgrades to composable upgrades

Add support for batched upgrades to composable upgrades

Some services (e.g ceph mon) require upgrading in batches (the old
upgrade architecture did the ceph mon upgrade one controller at a
time).  This interface enables doing the same, and over time we
can probably move more services into this interface (e.g when
services support rolling upgrades) to reduce downtime.

Change-Id: If581f301a5493ef33ac1386bdc22f9fca4f2544e
Partially-Implements: blueprint overcloud-upgrades-per-service