Add charm series data to metadata.yaml

This optimizes the publication process to the charm store, and
ensures that the charm code and version is consistent across all
supported Ubuntu series.

Change-Id: I65b02e8b07d5a98f4f352a3e4c00ce65cf54f281

metadata.yaml

@@ -7,6 +7,11 @@ tags:
   - openstack
   - storage
   - misc
+series:
+  - xenial
+  - yakkety
+  - trusty
+  - precise
 extra-bindings:
   public:
   admin: