Added installation of openstack horizon.

The image needs to be available on the target host. It can be built from:
ssh://git@git.webhosting.rug.nl:222/HPC/docker-horizon.git

roles/horizon/tasks/main.yml

@@ -0,0 +1,26 @@
+# Build and install a docker image for horizon.
+---
+- include: ../common/tasks/docker.yml
+
+- name: Make build and persistent directories
+  file:
+    path: "{{ item }}"
+    state: directory
+    mode: 0777
+  with_items:
+      - /srv/horizon
+
+- name: install service file.
+  template:
+    src: templates/horizon.service
+    dest: /etc/systemd/system/horizon.service
+    mode: 644
+    owner: root
+    group: root
+
+- command: systemctl daemon-reload
+
+- name: make sure service is started
+  systemd:
+    name: horizon.service
+    state: restarted