Initial scheduler support for instance_groups

Scheduler support for blueprint instance-group-api-extension

The patch set adds the following:
1. conductor support for getting instances via the instance_group uuid
2. conductor support for removing an instance from the instance_group
   members

In addition to this it replaces the 'group' hint with 'instance_group'.

DocImpact

Change-Id: I4a4febc2e85d4ffc8b1579d2673fa107171a3c5b
Merge "Initial scheduler support for instance_groups"