Support osd failure domain during install

[Chris Sanders]

While there is support for small / test clusters to run a single mon there isn't currently support to set failure domain to 'osd'. This provides the same limit as the mon, the minimum install needs at least 3 relations from ceph-osd to unblock.

Supporting a failure domain of 'osd' would remove remove the restriction similar to the mon setting that exists today. This becomes particularly useful once erasure coding is being used since a common use case for erasure coding is larger failure domains with less overhead.

A few notes, when setting the failure domain to osd the logic in the mon will need to check number of osd's not number of relations to allow other charms like ceph-fs and radosgw to receive keys and initialize.

[Edward Hope-Morley (hopem)]

Patch is here: https://review.openstack.org/#/c/530822/

[OpenStack Infra (hudson-openstack)]

Change abandoned by Chris Sanders (<email address hidden>) on branch: master
Review: https://review.opendev.org/530822
Reason: Due to inactivity I'm abandoning this change.