check_num_osds check not removed when nagios_check_num_osds is reset or set to false

[Paul Goins]

I recently filed a bug about the nagios_check_num_osds option's check failing on newer versions of ceph. This was https://pad.lv/1973832.

However, if I then try to disable the check to eliminate the bugged alerts for now, this also does not seem to work. The charm code will add the check when nagios_check_num_osds is set, but it will not remove the check when it is unset.