Allow ability to use image cache (#5)

* Add image_cache configuration

* Add image-cache example

* Adding a test for image-cache (based on single)

* Schedule cronjobs to do the cleaning/pruning, if image_cache enabled

* require glance services to be started, before enabling crons

* Update readme
diff --git a/tests/pillar/single_image_cache.sls b/tests/pillar/single_image_cache.sls
new file mode 100644
index 0000000..772dd12
--- /dev/null
+++ b/tests/pillar/single_image_cache.sls
@@ -0,0 +1,41 @@
+glance:
+  server:
+    enabled: true
+    version: liberty
+    workers: 1
+    database:
+      engine: mysql
+      host: localhost
+      port: 3306
+      name: glance
+      user: glance
+      password: password
+    registry:
+      host: 127.0.0.1
+      port: 9191
+    bind:
+      address: 127.0.0.1
+      port: 9292
+    identity:
+      engine: keystone
+      host: 127.0.0.1
+      port: 35357
+      user: glance
+      password: password
+      region: RegionOne
+      tenant: service
+      endpoint_type: internalURL
+    message_queue:
+      engine: rabbitmq
+      host: 127.0.0.1
+      port: 5672
+      user: openstack
+      password: password
+      virtual_host: '/openstack'
+    storage:
+      engine: file
+    image_cache:
+      enabled: true
+      enable_management: true
+      directory: /var/lib/glance/image-cache/
+      max_size: 21474836480