Allow setting up replication without providing the storage pool's name

I'll create this issue based on our recent discussion in Frankfurt.

When running the `microceph replication enable` command, a user has to provide the name of the storage pool (resource) for which replication should be enabled.
However this causes a chicken and egg situation in MicroCloud because a user would need to run through the following steps:

* Install MicroCloud (including MicroCeph) on cluster A and B
* Initialize MicroCloud on A and B with a storage pool on MicroCeph
* Enable replication in MicroCeph A using `microceph replication enable rbd --remote B
* Again go back to MicroCloud (LXD) and enable the replication on the already existing pool

From an LXD perspective it would be great if we could enable replication already when creating the pool to not require going back and forth.

This could be achieved by having the possibility of omitting the `` in the `microceph replication enable` command. In case the pool is omitted only the peers get setup without configuring the actual replication settings on a specific pool.

It would then be the responsibility of LXD to enable the replication using standard `rbd` commands.