rbd: fix #3286 add namespace support

rbd: fix #3286 add namespace support

This patch introduces support for Cephs RBD namespaces.

A new storage config parameter 'namespace' defines the namespace to be
used for the RBD storage.

The namespace must already exist in the Ceph cluster as it is not
automatically created.

The main intention is to use this for external Ceph clusters. With
namespaces, each PVE cluster can get its own namespace and will not
conflict with other PVE clusters.

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