pylint_plugins: add forbidden import checker

Add new pylint AST checker plugin which implements a check for imports
forbidden in IPA. Which imports are forbidden is configurable in pylintrc.

Provide default forbidden import configuration and disable the check for
existing forbidden imports in our code base.

Reviewed-By: Martin Basti <mbasti@redhat.com>

ipaplatform/base/services.py

@@ -93,11 +93,13 @@ class PlatformService(object):
     """
 
     def __init__(self, service_name, api=None):
+        # pylint: disable=ipa-forbidden-import
+        import ipalib  # FixMe: break import cycle
+        # pylint: enable=ipa-forbidden-import
         self.service_name = service_name
         if api is not None:
             self.api = api
         else:
-            import ipalib  # FixMe: break import cycle
             self.api = ipalib.api
             warnings.warn(
                 "{s.__class__.__name__}('{s.service_name}', api=None) "