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>

ipalib/config.py

@@ -48,7 +48,9 @@ from ipalib.constants import (
 )
 from ipalib import errors
 try:
+    # pylint: disable=ipa-forbidden-import
     from ipaplatform.tasks import tasks
+    # pylint: enable=ipa-forbidden-import
 except ImportError:
     tasks = None