mgr/DaemonServer: add js-output for "ceph osd safe-to-destroy"

mgr/DaemonServer: add js-output for "ceph osd safe-to-destroy"

E.g.:
 - case 1:
$ceph osd safe-to-destroy 0 1 2 -f json-pretty
Error EAGAIN: 12 pgs have unknown state; cannot draw any conclusions

 - case 2:
$ceph osd safe-to-destroy 0 1 2 -f json-pretty
{
    "safe_to_destroy": [
        0,
        1,
        2
    ],
    "active": [],
    "missing_stats": [],
    "stored_pgs": []
}

 - case 3:
$ceph osd safe-to-destroy 0 1 2 -f json-pretty
{
    "safe_to_destroy": [],
    "active": [
        0,
        1,
        2
    ],
    "missing_stats": [],
    "stored_pgs": []
}

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