Fix typo for self.list

Mistake was introduced in https://review.openstack.org/#/c/554950
Added an abstract method list to the base ResourceManager that
raises a notImplementedError if called (instead of using
abc.abstractmethod that would force the implementation of a list
method that might not be required for all resources)

Closes-Bug: #1820045

Change-Id: I0c69e1beeb452a95cc5988a462f733806afbfafc
(cherry picked from commit 0cd6b28292)

mistralclient/api/base.py

@@ -74,7 +74,15 @@ class ResourceManager(object):
         self.http_client = http_client
 
     def find(self, **kwargs):
-        return [i for i in self._list() if _check_items(i, kwargs.items())]
+        return [i for i in self.list() if _check_items(i, kwargs.items())]
+
+    def list(self):
+        """This is an abstract method
+
+        This is added here so that the find method gains some clarity.
+        It must be implemented by the child class in order to find to work
+        """
+        raise NotImplementedError("abstract method list must be implemented")
 
     @staticmethod
     def _build_query_params(marker=None, limit=None, sort_keys=None,