Support for scheduler hints for VM groups

This is part of the blueprint vm-ensembles. The patch introduces the
group as a scheduler hint.

The patch set adds group support for multi-VM deployment. This is achieved
as follows:
1. A new hint is added: group. This will contain the name and type for the
   group. At the moment only anti-affinity is supported. In the future
   we will add network proximity. It will be extended to <name>:<type>
2. In order to ensure that group policy will be honored for future VM
   deployments, the group is stored as system_metadata.
3. The anti affinity is implemented by a new filter called
   GroupAntiAffinityFilter.

DocImpact

Change-Id: I6ea2af5770b5ac4ff082b2a021d323ee38282205
Merge "Support for scheduler hints for VM groups"