Add CentOS support to the docker-storage-setup role

This let's us use the role on CentOS systems, as well as RHEL. In addition, it
installs docker and makes sure it's restarted (as opposed to just "started"
which has no effect when docker is already running).

roles/docker-storage-setup/tasks/main.yaml

@@ -25,5 +25,21 @@
     - ansible_distribution_version | version_compare('7.4', '<')
     - ansible_distribution == "RedHat"
 
+- block:
+    - name: create the docker-storage-setup config file for CentOS
+      template:
+        src: "{{ role_path }}/templates/docker-storage-setup-dm.j2"
+        dest: /etc/sysconfig/docker-storage-setup
+        owner: root
+        group: root
+        mode: 0644
+
+  # TODO(shadower): Find out which CentOS version supports overlayfs2
+  when:
+    - ansible_distribution == "CentOS"
+
+- name: Install Docker
+  package: name=docker state=present
+
 - name: start docker
-  service: name=docker state=started enabled=true
+  service: name=docker state=restarted enabled=true