We want to use the new resolver branch of pip today, and perhaps
develop in future too. This introduces the same basic change we did to
pbr's tests to test with a custom pip.
Change-Id: Ia2b1a9c7ee7a7bf32f404932fe7fe872ac402b0e
Depends-On: I7600fa20391bb83491d630b277bf01cc3a20727c