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: Ie586e319743ad647855d73c07c8b67cafa336e50

metadata.yaml

@@ -12,6 +12,11 @@ tags:
   - openstack
   - telemetry
   - misc
+series:
+  - xenial
+  - trusty
+  - precise
+  - yakkety
 extra-bindings:
   public:
   admin: