admin_socket: loop until the socket command succeeds

Sometimes the thing we're talking to is slow to start, or to register the
command we are running.  Loop in that case, at least for a while.

Signed-off-by: Sage Weil <sage@inktank.com>
task: mon_clock_skew_check: grab max-skew value from ceph-mon's config

Instead of relying on hardcoded values, obtain the max-skew default from
'ceph-mon --show-config-value mon_clock_drift_allowed' to match the mon's
expectation.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>