ci: stop using different images on the same run

ci: stop using different images on the same run

There is no point of using hosts running on atomic AND centos hosts. So
let's run containerized scenarios on Atomic only.

This solves this error here:

```
fatal: [client2]: FAILED! => {
    "failed": true
}

MSG:

The conditional check 'ceph_current_status.rc == 0' failed. The error was: error while evaluating conditional (ceph_current_status.rc == 0): 'dict object' has no attribute 'rc'

The error appears to have been in '/home/jenkins-build/build/workspace/ceph-ansible-nightly-luminous-stable-3.1-ooo_collocation/roles/ceph-defaults/tasks/facts.yml': line 74, column 3, but may
be elsewhere in the file depending on the exact syntax problem.

The offending line appears to be:

- name: set_fact ceph_current_status (convert to json)
  ^ here
```

From https://2.jenkins.ceph.com/view/ceph-ansible-stable3.1/job/ceph-ansible-nightly-luminous-stable-3.1-ooo_collocation/37/consoleFull#1765217701b5dd38fa-a56e-4233-a5ca-584604e56e3a

What's happening here is all the hosts excepts the clients are running atomic, so here: https://github.com/ceph/ceph-ansible/blob/master/site-docker.yml.sample#L62
The condition will skipped all the nodes excepts the clients, thus when running ceph-default, the task "is ceph running already?" is skipped but the task above needs the rc of the skipped task.
This is not an error from the playbook, it's a CI setup issue.

Signed-off-by: Sébastien Han <seb@redhat.com>