Add exception message to invalid cred field

This commit adds a new error message to the InvalidCredentials
exception raised when creating a new credentials object. Previously
it would just fail saying invalid credentials which wasn't
particularly useful for debugging what about the fields passed in was
invalid. The new error message will inform the user which key was the
cause of the error.

Change-Id: I49941321db7d6259080d706b687ac27173a3bffe

tempest_lib/auth.py

@@ -528,7 +528,8 @@ class Credentials(object):
             if key in self.ATTRIBUTES:
                 setattr(self, key, attr[key])
             else:
-                raise exceptions.InvalidCredentials
+                msg = '%s is not a valid attr for %s' % (key, self.__class__)
+                raise exceptions.InvalidCredentials(msg)
 
     def __str__(self):
         """Represent only attributes included in self.ATTRIBUTES"""