It is required to have an option for default crush ruleset

[Andrey Grebennikov]

In case of multi-tiered ceph cluster (ssd/spinners for example) there are expected to be multiple crush rules and the initial default rule won't apply.
It is required to be able to set up "osd pool default crush replicated ruleset" config option for Ceph monitors so that upon new pool creation via the charm's relation the new pool is automatically created with the desired rule and consequently on the desired set of OSDs.