This reduces the number of jobs from 48 to 24. The Mitogen part of the test
suite has been parameterized on the Linux container targets to be run against.
Both the Ansible tests & Mitogen tests now use the same source of truth to
control which targets to use: environment variable MITOGEN_TEST_DISTRO_SPECS.
This replaces the two mutually exclusive env vars DISTRO and DISTROS. I've
also removed vestgial traces of an unused env var MITOGEN_TEST_DISTRO.
Parameterization adapted from
https://eli.thegreenplace.net/2014/04/02/dynamically-generating-python-test-cases
refs #1058, #1059
Alex Willmer