By adding the keyring for RBD storage or the secret for CephFS ones, it
is possible to add an external Ceph cluster with only one API call.
Previously the keyring / secret file needed to be placed in
/etc/pve/priv/ceph/$storeID.{keyring,secret} manually.
Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>