Transition to python 3.x primary gates

This patch renames the python 3.x jobs to be the main gate jobs and
renames the python 2.7 gates to -py2- gates.

Job aliases are included for the transition period in other repositories.

This is in preparation for python2.7 EOL[1].

[1] https://legacy.python.org/dev/peps/pep-0373/

Change-Id: I79e20152826b946dbdf7baa5d8999aeee1558f43
2 files changed