Fix instance rename

As can be seen from the patch, we remove from the locking manager
inst.name. However, we do this right after cfg.RenameInstance, which
changes inst.name to the new name. So we need to remove old_name not
the new name.

The real question is why does the glm allow me to remove a not-existing
instance name...

The symptom before the patch was: run rename, then:
$ gnt-instance list
Unhandled protocol error while talking to the master daemon:
Caught exception: 'NoneType' object has no attribute 'primary_node'

Reviewed-by: imsnah