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/README.rst b/README.rst
index d372d2b..eafd56a 100644
--- a/README.rst
+++ b/README.rst
@@ -112,6 +112,19 @@
           virtual_host: '/openstack'
         ....
 
+Enable Glance Image Cache:
+
+.. code-block:: yaml
+
+    glance:
+      server:
+        image_cache:
+          enabled: true
+          enable_management: true
+          directory: /var/lib/glance/image-cache/
+          max_size: 21474836480
+      ....
+
 Enable auditing filter (CADF):
 
 .. code-block:: yaml