bluestore db and wal options read from ceph.conf

[James Page]

The bluestore DB and WAL size options are written to and then read from the ceph.conf for the charm. This is normally OK but when two ceph-osd applications are deployed to the same machine, the alternatives for ceph.conf are not resolved consistently across the deployment.

As a result the charm can read back a different value to that which it wrote to its copy of the ceph.conf under /var/lib/charm/>appname</ceph.conf.

These options are not actually used by ceph, just the charm, so the charm should actually just use them directly from juju config rather than through the ceph.conf which is just a layer on indirection to much.