Make install-docker compatible with centos

Allow custom docker_group which is needed on CentOS when
use_upstream_docker: false as is value is 'dockeroot'.

Assures that docker service is started, which is not always true
with all distributions.

Validates that we can talk with docker at the end.

Change-Id: I993c4ac35055a2a18b5b0fb32c8b230e04b7eb87

roles/install-docker/defaults/main.yaml

@@ -1,4 +1,5 @@
 use_upstream_docker: True
+docker_group: docker
 docker_update_channel: stable
 ubuntu_gpg_key: |
   -----BEGIN PGP PUBLIC KEY BLOCK-----

roles/install-docker/tasks/main.yaml

@@ -32,8 +32,26 @@
   user:
     name: "{{ ansible_user }}"
     groups:
-      - docker
+      - "{{ docker_group }}"
     append: yes
 
-- name: reset ssh connection to pick up docker group
+- name: Assure docker service is running
+  become: yes
+  service:
+    name: docker
+    enabled: yes
+    state: started
+
+- name: Correct group ownership on docker sock
+  become: yes
+  file:
+    path: /var/run/docker.sock
+    group: "{{ docker_group }}"
+
+- name: Reset ssh connection to pick up docker group
   meta: reset_connection
+
+- name: Validate ability to talk with docker
+  command: docker ps
+  args:
+    warn: no