Pin func test requirements to stable/train; the last stable release
with Python 2 support. Switch to the python-jobs-no-constraints
template to avoid using master constraints file.
Only build focal on bionic, other debootstraps are too old.
Remove pip-and-virtualenv testing as we are moving to plain images.
The tripleo-buildimage-jobs are unstable; see linked bug.
Add a note that this is the last Python 2 release.