Fix a few flake8 issues from previous changes

maestro/entities.py

@@ -175,7 +175,7 @@ def get_image_ids(self):
         images = {}
         for image in self._backend.images():
             tags = image.get('RepoTags', [])
-            if not tags or tags is '<none>:<none>':
+            if not tags or tags == '<none>:<none>':
                 continue
             for tag in image['RepoTags']:
                 images[tag] = image['Id']

maestro/exceptions.py

@@ -57,29 +57,31 @@ def __str__(self):
         return '{}: {}'.format(self.container.name, self.message)
 
 
-class InvalidPortSpecException(MaestroException):
+class InvalidPortSpecException(EnvironmentConfigurationException):
     """Error thrown when a port spec is in an invalid format."""
     pass
 
 
-class InvalidLifecycleCheckConfigurationException(MaestroException):
+class InvalidLifecycleCheckConfigurationException(
+        EnvironmentConfigurationException):
     """Error thrown when a lifecycle check isn't configured properly."""
     pass
 
 
-class InvalidRestartPolicyConfigurationException(MaestroException):
+class InvalidRestartPolicyConfigurationException(
+        EnvironmentConfigurationException):
     """Error thrown when a restart policy isn't configured properly."""
     pass
 
 
-class InvalidVolumeConfigurationException(MaestroException):
+class InvalidVolumeConfigurationException(EnvironmentConfigurationException):
     """Error thrown when a volume binding isn't configured properly."""
 
 
-class InvalidAuditorConfigurationException(MaestroException):
+class InvalidAuditorConfigurationException(EnvironmentConfigurationException):
     """Invalid configuration of one of the specified auditors."""
 
 
-class InvalidLogConfigurationException(MaestroException):
+class InvalidLogConfigurationException(EnvironmentConfigurationException):
     """Error thrown when a log_driver or log_opt is in an invalid format."""
     pass