Add linux.storage.loopback state

This state allows to configure loopback device(s). This isn't meant to
be used in production but offers a cheap way to test Cinder with LVM
volumes.
diff --git a/README.rst b/README.rst
index b2cf09f..1c7d79b 100644
--- a/README.rst
+++ b/README.rst
@@ -693,6 +693,17 @@
           multipath:
             enabled: false
 
+Linux with local loopback device
+
+.. code-block:: yaml
+
+    linux:
+      storage:
+        loopback:
+          disk1:
+            file: /srv/disk1
+            size: 50G
+
 External config generation
 --------------------------