We need the integration gate to work with both trusty (python3.4) and
xenial (python3.5). Rather than hard code the full python binary just
use python 3 and let the OS mange this for us. This is less than ideal
but it's the best we can do right now to unblock the gate.
We also address a short term issue with out upper-constraints.txt files
and manually copy the constraints from 3.4 to 3.5
Related-Bug: 1620436
Change-Id: I6acabf86933b7a7bce7baf44e6b512e7d0d2f6db