Ceph

add a check_config() type method to the config observer and init to verify that important config tunables (like timeouts and intervals for elections, paxos, leases, clock skew) obey the necessary constraints for correctness.

checks should include these tunables, at a minimum:

- mon_accept_timeout
- mon_lease_ack_timeout
- mon_leas_renew_interval

See similar pull request for checks in the osd: https://github.com/ceph/ceph/pull/1964