rados block plugin update

rados block plugin update

config file
-----------
rbd: rbdtest
        monhost 10.3.94.27:6789;10.3.94.28:6789;10.3.94.29:6789
        pool pool2
        username admin
        authsupported cephx;none
        content images

key for user admin must be specified in:
---------------------------------------
/etc/pve/priv/ceph/storeid.username.key

/etc/pve/priv/ceph/rbdtest.admin.key   for this exemple.

(so we can have multiple users key by storage and multiple storage with same username)

I change "sub path" in Storage.pm to pass $storeid as third argument.

I rename property id to username, to be more clear.

removal of rbd_ prefix.

removal of trailing whitespace

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