NRPE: mon is low on available space warning

[Adam Dyess]

The cluster health status goes to HEALTH_WARN on all the ceph-mon when any of the monitor's disk are below a threshold that cannot be adjusted by the charm.

Consider 3 ceph-mons /0, /1, and /2
Each has a subordinate nrpe-charm checking disks at W25 C20 K5

But the default for a ceph-mon disk is 30%

https://docs.ceph.com/docs/master/rados/configuration/mon-config-ref/
mon data avail warn

Since the charm doesn't change this config, the cluster gets a warning when any one of the mon unit's disk drops below 30% storage remaining.

Any cluster warning is treated as a critical error -- defeating the purpose of the NRPE.

Please make this config possible in the charm so i can adjust the mon data avail warn to 5%.
Bonus points will be awared for using the config from the NRPE subordinate