task: ceph: don't wait for 'healthy' if 'wait-for-healthy' is false.

This new config option obviously defaults to 'true' in order to not only
maintain compatibility, but because it makes sense.

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>
task: ceph_manager: add 'get_mon_health' function

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>
task: mon_clock_skew_check.py: Check for clock skews on the monitors

Will run for as long as teuthology runs. By default, fails if any clock
skews higher than 0.05 seconds are detected, but will only fail when the
teuthology run finishes and after reporting a list of all the detected
skews.

Accepted options:

 interval     amount of seconds to wait in-between checks. (default: 30.0)
 max-skew     maximum skew, in seconds, that is considered tolerable
              before issuing a warning. (default: 0.05)
 expect-skew  'true' or 'false', to indicate whether to expect a skew
              during the run or not. If 'true', the test will fail if no
              skew is found, and succeed if a skew is indeed found; if
              'false', it's the other way around. (default: false)
 never-fail   Don't fail the run if a skew is detected and we weren't
              expecting it, or if no skew is detected and we were
              expecting it. (default: False)

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>
Merge remote-tracking branch 'gh/wip-3633'