Instance groups: add method get_by_hint

In order to support backward compatibility a new method
'get_by_hint' has been added. This method will check if the
hint is a uuid and if no it will return get_by_uuid else it
will return get_by_name.

Change-Id: I728bf3454dc5a5367e21f242281719c1a967269a
Partial-bug: #1303360
Merge "Instance groups: add method get_by_hint"
Merge commit 'refs/changes/38/84138/2' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch