Medium: agent: detect if the stonith-timeout is too low (bnc#841010)

If the stonith-timeout (minus some overhead for pacemaker internals) is
lower than then msgwait, sbd will never be able to succeed but always
timeout from pacemaker's perspective - from the user's point of view,
the poison pill will have been delivered and confusion will ensue.

This changeset detects this mismatch.