support stevedore >= 0.14

the previous mocks made some very specific assumptions about the
behavior of an internal private method in stevedore, which changed
in 0.14. Make the mocks more flexible so they work regardless.

Change-Id: I5063c652c705fd512f90ff3897a4c590f7ba7c02
(cherry picked from commit 3992cfa3463fa7ab72a746cae38f7e212ad1a00a)