core: revert xmlrpc fallback

Previous commit (Change-Id: Ie6f48bec60b520c089f326f8c5e79aec288ff3d6)
introduced a fallback mechanism to support getting
the proper non-operational reason when adding old hosts to 3.5 cluster.
I encountered some issues with the fix, so reverting the Install logic
until further examination.

Change-Id: Ib45a6e828e459f982c0c77ca778f2b83e97a881b
Signed-off-by: Oved Ourfali <oourfali@redhat.com>