qa: use real entity for config_get

qa: use real entity for config_get

So this changed in Octopus/Pacific to allow

    ceph config get mds foo

but it doesn't work in Nautilus which expects a real entity:

https://pulpito.ceph.com/yuriw-2020-08-07_15:03:29-kcephfs-wip-yuri4-testing-2020-08-07-1350-nautilus-distro-basic-smithi/5308647/

    2020-08-07T18:56:49.830 INFO:teuthology.orchestra.run.smithi106.stderr:2020-08-07 18:56:49.833 7fbe0d7fa700  1 -- 172.21.15.106:0/369883181 <== mon.1 v2:172.21.15.106:3300/0 7 ==== mon_command_ack([{"prefix": "config get", "who": "mds", "key": "mds_min_caps_per_client"}]=-22 unrecognized entity 'mds' v1) v1 ==== 131+0+0 (secure 0 0 0) 0x7fbdf8000940 con 0x7fbdf40064c0
    2020-08-07T18:56:49.830 INFO:teuthology.orchestra.run.smithi106.stderr:Error EINVAL: unrecognized entity 'mds'

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>