mon/OSDMonitor: "osd pool application ls" support

mon/OSDMonitor: "osd pool application ls" support

It would be a pain if we have to call 'ceph osd dump --format=json-pretty'
to find out these each time...

Demo output:
(1) ceph osd pool application get
{
    "cephfs_data_b": {
        "cephfs": {}
    },
    "cephfs_metadata_a": {
        "cephfs": {}
    },
    "test_pool": {
        "rbd": {
            "test": "me"
        }
    }
}

(2) ceph osd pool application get test_pool
{
    "rbd": {
        "test": "me"
    }
}

(3) ceph osd pool application get test_pool rbd
{
    "test": "me"
}

(4) ceph osd pool application get test_pool rbd test
me

Fixes: http://tracker.ceph.com/issues/20976
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>