Handle special case where an OP_INSTANCE_CREATE job which uses the
opportunistic locking feature fails. This failure is probably due to the
fact that all nodes may be already locked. In this case, make
snf-dispatcher retry the failed job with opportunistic locking turned
off. Also, make snf-ganeti-eventd include in the message the input of
the failed job.

Finally, add setting 'GANETI_USE_OPPORTUNISTIC_LOCKING' to turn on/off
the use of opportunistic locking feature.