add-disk and zap-disk actions don't work for mounted filesystem paths

When trying to replace a failed disk on an environment with 19.04 charms and using osd-devices=/path/to/mountpoint1 /path/to/mountpoint2 ..., we cannot add-disk /path/to/mountpoint2 if it had to be rebuilt due to hardware replacement.

The process would typically be:

juju run-action --wait ceph-osd/X zap-disk devices=/path/to/mountpoint2 i-really-mean-it=yes
* replace disk, reformat, and mount *
juju run-action --wait ceph-osd/X add-disk osd-devices=/path/to/mountpoint2

unfortunately, zap-disk action bombs out immediately if devices is not a block device.

zap-disk is the only way in the charm to remove that path from the kv() db of known/configured disks.

Suggest zap-disk have the ability to reduce the db disks values by the path provided at a very minimum so that add-disk doesn't skip the re-add of the disk.

Current workaround to allow for add-disk on already-configured path:

1. login to unit
2. cd /var/lib/juju/agents/unit-ceph-osd-X/charm
3. apt-get install sqlite
4. sudo sqlite3 .unit-state.db
5. enter the following in sqlite prompt:
    select data from kv where key='osd-devices';
    *capture output for update command*
    update kv set data='<list construct minus the path to zap>' where key='osd-devices';
    select data from kv where key='osd-devices';
    *validate setting is as expected*
    .quit