Allow setting `ceph.osd.pool_size` on pool creation

[masnax]

At least with MicroCeph, OSD pools will default to a replication size of 3. That means if there are fewer than that many OSDs available for replication, LXD will fail to create the pool when running lxc storage create pool ceph. The command will block as the OSD pool will try to replicate unsuccessfully during initialization.

In such cases (admittedly usually testing setups), it may be useful to specify ceph.osd.pool_size to apply as an initial argument when creating the pool. This may also be useful in the LXD test suite as well, where we need to set microceph.ceph config set global osd_pool_default_size 1 to change the default behaviour for all OSD pools in the test suite to disable replication.

Somewhat related to the MicroCloud OSD replication discussions going on, but MicroCloud would not leverage this capability in LXD because it also needs to be able to update the value when adding new cluster members, and it seems the ceph storage pool has no update implementation either.