tests: add mimic support for test_rbd_mirror_is_up()

tests: add mimic support for test_rbd_mirror_is_up()

prior mimic, the data structure returned by `ceph -s -f json` used to
gather information about rbd-mirror daemons looked like below:

```
  "servicemap": {
    "epoch": 8,
    "modified": "2018-07-05 13:21:06.207483",
    "services": {
      "rbd-mirror": {
        "daemons": {
          "summary": "",
          "ceph-nano-luminous-faa32aebf00b": {
            "start_epoch": 8,
            "start_stamp": "2018-07-05 13:21:04.668450",
            "gid": 14107,
            "addr": "172.17.0.2:0/2229952892",
            "metadata": {
              "arch": "x86_64",
              "ceph_version": "ceph version 12.2.5 (cad919881333ac92274171586c827e01f554a70a) luminous (stable)",
              "cpu": "Intel(R) Core(TM) i7-4870HQ CPU @ 2.50GHz",
              "distro": "centos",
              "distro_description": "CentOS Linux 7 (Core)",
              "distro_version": "7",
              "hostname": "ceph-nano-luminous-faa32aebf00b",
              "instance_id": "14107",
              "kernel_description": "#1 SMP Wed Mar 14 15:12:16 UTC 2018",
              "kernel_version": "4.9.87-linuxkit-aufs",
              "mem_swap_kb": "1048572",
              "mem_total_kb": "2046652",
              "os": "Linux"
            }
          }
        }
      }
    }
  }
```

This part has changed from mimic and became:
```
  "servicemap": {
    "epoch": 2,
    "modified": "2018-07-04 09:54:36.164786",
    "services": {
      "rbd-mirror": {
        "daemons": {
          "summary": "",
          "14151": {
            "start_epoch": 2,
            "start_stamp": "2018-07-04 09:54:35.541272",
            "gid": 14151,
            "addr": "192.168.1.80:0/240942528",
            "metadata": {
              "arch": "x86_64",
              "ceph_release": "mimic",
              "ceph_version": "ceph version 13.2.0 (79a10589f1f80dfe21e8f9794365ed98143071c4) mimic (stable)",
              "ceph_version_short": "13.2.0",
              "cpu": "Intel(R) Xeon(R) CPU           X5650  @ 2.67GHz",
              "distro": "centos",
              "distro_description": "CentOS Linux 7 (Core)",
              "distro_version": "7",
              "hostname": "ceph-rbd-mirror0",
              "id": "ceph-rbd-mirror0",
              "instance_id": "14151",
              "kernel_description": "#1 SMP Wed May 9 18:05:47 UTC 2018",
              "kernel_version": "3.10.0-862.2.3.el7.x86_64",
              "mem_swap_kb": "1572860",
              "mem_total_kb": "1015548",
              "os": "Linux"
            }
          }
        }
      }
    }
  }
```

This patch modifies the function `test_rbd_mirror_is_up()` in
`test_rbd_mirror.py` so it works with `mimic` and keeps backward compatibility
with `luminous`

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 09d795b5b737a05164772f5e3ba469577d605344)