Auto switch to scale from add_unit on container based models

Fixes #651

Author: cderici

examples/charmhub_deploy_k8s.py

@@ -24,9 +24,12 @@ async def main():
         )
 
         print('Waiting for active')
-        await model.block_until(
-            lambda: all(unit.workload_status == 'active'
-                        for unit in application.units))
+        await model.wait_for_idle(status="active")
+
+        # when run on a container based model it should auto-switch to
+        # scale instead of failing with JujuError
+        await application.add_unit(count=2)
+        await application.destroy()
 
     finally:
         print('Disconnecting from model')

juju/application.py

@@ -131,6 +131,11 @@ async def add_unit(self, count=1, to=None):
             If None, a new machine is provisioned.
 
         """
+
+        if self.model.info.type_ == 'caas':
+            log.warning('adding units to a container-based model not supported, auto-switching to scale')
+            return await self.scale(scale_change=count)
+
         app_facade = self._facade()
 
         log.debug(