Fix restoration of old confluent db

Old confluent DB may have None in role. This is no longer
allowed.  Restore such entries by coercing them to 'Administrator'
which is how old confluent treated such users.

confluent_server/confluent/config/configmanager.py

@@ -1625,6 +1625,8 @@ class ConfigManager(object):
         name = confluent.util.stringify(name)
         if name in self._cfgstore['users']:
             raise Exception("Duplicate username requested")
+        if role is None:
+            role = 'Administrator'
         for candrole in _validroles:
             if candrole.lower().startswith(role.lower()):
                 role = candrole