Fix: schemas: Use fence-event-2.15.rng
The move to fence-event-2.15.rng (46dd9b7) did not include updates to
the crm_mon and stonith_admin schemas. As a result, the API result fails
to validate if it contains a fence event with an exit-reason.
Resolves: RHBZ#2086230
Signed-off-by: Reid Wahl <nrwahl@protonmail.com>