Avoiding conflicting CRUSH bucket keys

As of Ceph Luminous, bucket keys within the CRUSH map must be
unique; The root bucket is always called 'default' so remap
Juju and configuration provided bucket keys to
'default-{row,rack}' ensuring that keys are unique.

Change-Id: I7fa3dd9e001cca40e678e8983a1d7ed19d51e2fe
Closes-Bug: 1729911