Add InstanceGroup.get_by_name()

Legacy behavior of the 'group' scheduler hint was that the value was
treated as a name for the group.  To preserve the old behavior, the
InstanceGroup object needs to be able to look up a group by name.

Since we're so close to doing an RC, just use the db API methods
available now but leave a note that this should be optimized later
with a db.api query that looks up the group by name.

Change-Id: I1b8c72b29a9a44a171aaa231aeb3533ccf41f7c2
Partial-bug: #1296913
Merge "Add InstanceGroup.get_by_name()"