ceph pools: allow to create erasure code pools

ceph pools: allow to create erasure code pools

To use erasure coded (EC) pools for RBD storages, we need two pools. One
regular replicated pool that will hold the RBD omap and other metadata
and the EC pool which will hold the image data.

The coupling happens when an RBD image is created by adding the
--data-pool parameter. This is why we have the 'data-pool' parameter in
the storage configuration.

To follow already established semantics, we will create a 'X-metadata'
and 'X-data' pool. The storage configuration is always added as it is
the only thing that links the two together (besides naming schemes).

Different pg_num defaults are chosen for the replicated metadata pool as
it will not hold a lot of data.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>