Fixed certain bugs on rest admin APIs

Signed-off-by: Babu Shanmugam <anbu@enovance.com>
Removed the check for parameter validation in op_get()

Signed-off-by: Babu Shanmugam <anbu@enovance.com>
ceph_json: don't try to parse NULL buffer

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>