Add rados block plugin storage

Add rados block plugin storage

This add rados plugin storage.

We need rbd command from ceph repository (ceph-common package) to create/delete volumes,I didn't have add depends for the moment.

/etc/pve/storage.cfg sample config

rbd: rbdtest
        rbd_monhost 10.3.94.27:6789;10.3.94.28:6789;10.3.94.29:6789
        rbd_pool pool2
        rbd_id admin
        rbd_key AQAmOcZPwNY7GRAAuvJjVAKIm1r3JKqLCa4LGQ==
        rbd_authsupported cephx;none
        content images

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>