tests: add more memory for rgw_multsite scenarios

tests: add more memory for rgw_multsite scenarios

Adding more memory to VMs for rgw_multisite scenarios could avoid this error
I have recently hit in the CI:

(It is worth it to set 1024Mb since there is only 2 nodes in those
scenarios.)

```
fatal: [osd0]: FAILED! => {
    "changed": false,
    "cmd": [
        "docker",
        "run",
        "--rm",
        "--entrypoint",
        "/usr/bin/ceph",
        "docker.io/ceph/daemon:latest-luminous",
        "--version"
    ],
    "delta": "0:00:04.799084",
    "end": "2018-10-29 17:10:39.136602",
    "rc": 1,
    "start": "2018-10-29 17:10:34.337518"
}

STDERR:

Traceback (most recent call last):
  File "/usr/bin/ceph", line 125, in <module>
    import rados
ImportError: libceph-common.so.0: cannot map zero-fill pages: Cannot allocate memory
```

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>