Fixes namespace issue

This patch renames the mistral directory to mistral_lib to ensure
an action would not hit a namespace issue when attempting to import.
e.g. "from mistral import context".

Change-Id: I984b6a9569aef9e9f52fb3c037ff133081b7d933

mistral_lib/tests/test_mistral_actions_api.py

@@ -14,7 +14,7 @@
 Stub module for Custom Actions API tests.
 """
 
-from mistral.tests import base
+from mistral_lib.tests import base
 
 
 class TestActionsAPI(base.TestCase):