Fix unimplemented abstractmethod

In [0], castellan added a new abstractmethod to the base class which
SaharaKeyManager inherits from. We need to override this abstractmethod
with our own dummy function or else face TypeError.

[0] https://review.openstack.org/#/c/418019/

Change-Id: Ibac3e8c9086f000029bcb6001643edf24a3951ad
Closes-Bug: #1709313

sahara/service/castellan/sahara_key_manager.py

@@ -74,3 +74,10 @@ class SaharaKeyManager(km.KeyManager):
         perform any external actions. therefore, it won't change anything.
         """
         pass
+
+    def list(self, *args, **kwargs):
+        """list all managed keys
+
+        current implementation of the key manager does not utilize this
+        """
+        pass