monitor_service's "passive" parameter should be a string, not a boolean

This fixes a regression introduced in I193da61a7.

monitor_service in nagios.pp does a string literal comparison with 'true' to
determine whether the check is active or passive. a boolean true doesn't match,
so $passive evaluates to whatever is specified as default.

nagios.pp should be refactored, but that's for another patch.

Change-Id: I6d0187cf209751920f1ed5de40d909f75fac2f70