Support for specifying WAL/DB device parameters for MicroCeph OSDs from the MicroCloud init (Manual or Preseed)

[pamudithaA]

Issue:

MicroCeph improves performance for spinning disks (HDDs) by using a separate SSD partition for the write-ahead log (WAL) and the BlueStore database. This is explained in the microceph disk add.
However, the current MicroCloud init flow (both manual and preseed file) does not support specifying the additional configuration parameters. This prevents us from creating Ceph clusters that use a separate WAL/DB partitions using the init flow.

Workaround used for the current use case:

Currently we are initializing the MicroCloud using the preseed file with the minimum number of OSDs. Then once the MicroCloud cluster is ready, we are adding the disks using microceph disk add </dev/sdX> --wipe --db-device </dev/sdYi> --db-wipe command.

[roosterfish]

Linking this story here from LXD which is slightly related as it will allow the proper usage of bcache devices for MicroCeph canonical/lxd#16347.