Add series support and revert previous commit that included build charm rather than layer

Change-Id: I554fbe51c5ec61f4ac39b70f5d6123432ea0bc02
Signed-off-by: Adam Israel <adam.israel@canonical.com>

vpe-router/metadata.yaml

@@ -3,6 +3,9 @@ maintainers:
   - Marco Ceppi <marco.ceppi@canonical.com>
   - Adam Israel <adam.israel@canonical.com>
 summary: setup a virtualized PE Router with GRE tunnels
+series:
+  - trusty
+  - xenial
 description: | 
   this charm, when deployed and configured, will provide a secure virtualized
   provider edge router.