mon/OSDMonitor: fix "ceph osd pool get rbd all --format=json-pretty"

mon/OSDMonitor: fix "ceph osd pool get rbd all --format=json-pretty"

Two problems:
(1) MIN_WRITE_RECENCY_FOR_PROMOTE is a tier-only option.
(2) should automatically filter out unset pool options, otherwise it will
    keep outputing rubbish:

{
    "pool": "rbd",
    "pool_id": 3,
    "min_write_recency_for_promote": 0
}
{
    "pool": "rbd",
    "pool_id": 3,
    "fast_read": 0
}
{
    "pool": "rbd",
    "pool_id": 3
}
{
    "pool": "rbd",
    "pool_id": 3
}
{
    "pool": "rbd",
    "pool_id": 3
}
{
    "pool": "rbd",
    "pool_id": 3,
    "csum_type": "???"
}

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