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.

Related-Bug: #1273455

Addapted from master & stable/havana backports

Change-Id: I5063c652c705fd512f90ff3897a4c590f7ba7c02