The common version of get-pip.py no longer supports python3.6 or older.
Devstack has amended it's use of this file to use a stable version of
the installer for python3.6 [0]. Pre-cache this version in our images,
too, so that it can be consumed in the CI.
[0] https://review.opendev.org/#/q/Iab2c391d5388461fe9e9037cee81884ce8032e72
Change-Id: Ied14dc7188e27e2a4b144d597b696ee85f25721d