mon: fix inaccurate mon-num counting of 'ceph features' command

mon: fix inaccurate mon-num counting of 'ceph features' command

Monitors need to talk to each other, which causes the inaccurate
mon-num counting logic of 'ceph features' command.
E.g.:
(1) By specifying 2 monitors, you get 4 from 'ceph features':
    "mon": {
        "group": {
            "features": 2305244844532236283,
            "release": "luminous",
            "num": 4
        }
    },
(2) By specifying 3 monitors, you get 9 from 'ceph features':
   "mon": {
        "group": {
            "features": 2305244844532236283,
            "release": "luminous",
            "num": 9
        }
    },

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>