py3: Switch packages to Python 3 for train and later
Switch package install to Python 3 for OpenStack Train and later.
When upgrading, remove any python-* packages that were explicitly
installated and then autoremove --purge any dependencies that are
no longer required.
This patch also includes the following related changes:
* Use the common files package, swift, rather than python-swift
when using a package name to determine release.
Also add OS_* to tox.ini to allow functional tests to execute.
Ensure that EOL releases are removed from metadata
and tests, and that the current dev release is enabled
in metadata. Dev release tests are enabled separately
because of chickens and eggs.