cache get-pip.py

One of the few network interactions that we still have in devstack
gate is downloading get-pip.py from github (it's not even in the
pypi cdn). This fails a few times a day on average.

This is avoidable by adding get-pip.py to the devstack cache during
node prep.

Closes-Bug: #1299176

Change-Id: I72a75f27a416e9a4cc98b42c2702293676ca0e2d
Merge "cache get-pip.py"