Updates server_metadata for 2.0.0

- Switch to using proxy layer calls
- Return whole server resource instead of only metadata map
- Swap the name and server aliased parameters
- Add a server_metadata test role
- Ensure check mode returns tentatively updated metadata
- Remove server_id return value

Change-Id: Id63e6a80d1b8e3574911016fec792f00b63f1524

.zuul.yaml

@@ -103,6 +103,7 @@
         security_group
         security_group_rule
         server
+        server_metadata
         server_volume
         stack
         subnet