docker: fix mkdir path for all the distros

On ubntu systems mkdir is in /bin where on atomic it is /usr/bin/.
We use the shell built-in function "command" to find its right location.

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

roles/ceph-mon/templates/ceph-mon.service.j2

@@ -5,7 +5,7 @@ After=docker.service
 [Service]
 EnvironmentFile=-/etc/environment
 ExecStartPre=-/usr/bin/docker rm %i
-ExecStartPre=/usr/bin/mkdir -p /etc/ceph /var/lib/ceph/mon
+ExecStartPre=$(command -v mkdir) -p /etc/ceph /var/lib/ceph/mon
 ExecStart=/usr/bin/docker run --rm --name %i --net=host \
    {% if not mon_containerized_deployment_with_kv -%}
    -v /var/lib/ceph:/var/lib/ceph \