ResourceGroup allow update of resource_def

Currently the group is replaced if the resource_def is updated,
but instead we should allow updates and update the underlying
nested stack instead.  This allows properties to be changed
such that non-replacement updates can be performed on the
resource-group members.

Change-Id: I471b7ebe71ed262a8b0e2c971a32afc7062699cb
Closes-Bug: #1396533
Closes-Bug: #1380612
Merge "ResourceGroup allow update of resource_def"