Add multi-series back into metadata.yaml

This is necessary because the aodh charm was previously
manually pushed into the charm store as a multi-series
charm.

Change-Id: I1e3d55f2f4bcd578e7f77c81fbbcecc4095c149c

src/metadata.yaml

@@ -9,6 +9,9 @@ description: |
  overall system.
  .
  Aodh provides the Alarming service as part of OpenStack telemetry.
+series:
+  - xenial
+  - trusty
 requires:
   mongodb:
     interface: mongodb