python-cephfs: use rados namespace for data isolation.

python-cephfs: use rados namespace for data isolation.

As cephfs dir layout supports rados namespace in jewel, it would be
good to use rados namespace instead of pool, since it is free,
we always enforce namespace isolation for volumes.

The meaning of "data_isolated" flag changed a bit, it's no longer used
for security isolation, referring to physical isolation
(i.e. potentially using different OSDs).

Also, (de)authorize based on namespace.

Signed-off-by: Xiaoxi Chen <xiaoxchen@ebay.com>
(cherry picked from commit 0e1d013ea690c18f7fa6e2a19500f854a9949091)