Add validation for --revision flag to require --channel flag

This is needed because in libjuju we default to latest/stable if no
channel is specified. It is dangerous if we don't add this check
because without it the libjuju would deploy the wrong revision without
an exception if --revision is used but no --channel is given.

Author: cderici

juju/model.py

@@ -499,6 +499,8 @@ async def resolve(self, url, architecture, app_name=None, channel=None, series=N
         the charmhub API is required to correctly determine the charm url and
         underlying origin.
         """
+        if revision and not channel:
+            raise JujuError('ERROR specifying a revision requires a channel for future upgrades. Please use --channel')
 
         ch = Channel('latest', 'stable')
         if channel is not None: