Fix new sqlalchemy-migrate jobs

Change Iea900c69d35a9c083147f115051d6656d8ea5d24 added two additional
jobs for sqlalchemy-migrate to run tests with two different SQLAlchemy
versions (this is implemented as two separate tox envs). Unfortunately,
it incorrectly uses run-tox.sh script, so these new jobs always fail.

This patch fixes those sqlalchemy-migrate jobs and adds them to the
experimental pipeline first to ensure they work correctly.

Change-Id: Ie3c2f39ef8b0faa029f5055b5fc54897272dd906
Merge "Fix new sqlalchemy-migrate jobs"