rdbms: better handle a non-existing "defaultGroup" in LoadBalancer
If the specified default group does not have a corresponding server load map
defined, then ignore it and use GROUP_GENERIC.
Also, consolidate the group load and group reader index code for simplicity
Bug: T228104
Change-Id: Ic8bf9a3ebcbffb81fb14d7b1787a2adb97ac525d
(cherry picked from commit 32df79f3d1966f41d6ddfb954a061361827499c2)