Get series from deployed app instead of metadata when charm upgrade

Fixes #668

Charmhub is modeling the charm data separately from where the charm is
deployed, therefore the series is not a part of the metadata anymore. So
 instead of getting the series from the metadata we now get it
 from the deployed application first, then we try the
 model config, if everything fails then we look at the metadata as a
 last resort (mostly to keep backwards compatibility).

Author: cderici

juju/application.py

@@ -339,6 +339,18 @@ async def unexpose(self, exposed_endpoints=None):
         log.debug("Unexposing %s", self.name)
         return await app_facade.Unexpose(application=self.name)
 
+    async def get_series(self):
+        """Return the series on which the application is deployed
+
+        :return: str series
+        """
+        app_facade = self._facade()
+
+        log.debug(
+            'Getting series for %s', self.name)
+
+        return (await app_facade.Get(application=self.name)).series
+
     async def get_config(self):
         """Return the configuration settings dict for this application.
         """
@@ -728,9 +740,12 @@ async def local_refresh(
         charm_dir = path.expanduser().resolve()
         model_config = await self.get_config()
 
-        series = await get_charm_series(charm_dir, self.model)
+        series = (
+            await self.get_series() or
+            self.model.info.get('default-series', '') or
+            await get_charm_series(charm_dir, self.model)
+        )
         if not series:
-            model_config = await self.get_config()
             default_series = model_config.get("default-series")
             if default_series:
                 series = default_series.value

tests/integration/upgrade-charm/metadata.yaml

@@ -1,5 +1,5 @@
 name: ubuntu
-series: ["focal"]
+series: ["bionic", "focal"]
 summary: "test"
 description: "test"
 maintainers: ["test"]