additional checks are not removed

[Giuseppe Petralia]

This is focal ussuri, ceph octopus and charm ceph-mon stable rev. 61

When updating the additional checks the existing ones are not removed.

Step to reproduce:

1. Add additional checks:
   juju config ceph-mon nagios_additional_checks="{'ceph_slow_requests': 'slow requests', 'ceph_nearfull' : 'nearfull osd' }"

2. New checks are added:

ll /etc/nagios/nrpe.d
...
-rw-r--r-- 1 root root 348 Nov 28 10:45 check_ceph-ceph_nearfull.cfg
-rw-r--r-- 1 root root 359 Nov 28 10:45 check_ceph-ceph_slow_requests.cfg
...

3. Update additional checks with:
   juju config ceph-mon nagios_additional_checks="{'slow-requests': 'slow requests', 'nearfull' : 'nearfull osd' }"

4. Now the checks are duplicated:

ll /etc/nagios/nrpe.d
...
-rw-r--r-- 1 root root 348 Nov 28 10:45 check_ceph-ceph_nearfull.cfg
-rw-r--r-- 1 root root 359 Nov 28 10:45 check_ceph-ceph_slow_requests.cfg
-rw-r--r-- 1 root root 338 Nov 28 10:48 check_ceph-nearfull.cfg
-rw-r--r-- 1 root root 349 Nov 28 10:48 check_ceph-slow-requests.cfg
...