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/templates/horizon.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Openstack Glance Container
+After=docker.service
+Requires=docker.service
+
+[Service]
+TimeoutStartSec=0
+Restart=always
+ExecStartPre=-/usr/bin/docker stop %n
+ExecStartPre=-/usr/bin/docker rm %n
+ExecStart=/usr/bin/docker run --name %n \
+  -e "MEMCACHED_SERVER={{ hostvars[groups['keystone'][0]]['ansible_default_ipv4']['address'] }}" \
+  -e "OPENSTACK_HOST={{ hostvars[groups['keystone'][0]]['ansible_default_ipv4']['address'] }}" \
+  -p 80:80 \
+  hpc/horizon
+
+[Install]
+WantedBy=multi-user.target