task: mon_clock_skew_check: add option to run at least one timecheck

  at-least-once          Runs at least once, even if we are told to stop.
                         (default: True)
  at-least-once-timeout  If we were told to stop but we are attempting to
                         run at least once, timeout after this many
                         seconds. (default: 300)

Fixes: #3854

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>