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.
Change-Id: I121af845bf11c22052479a497b196a4670021256
fb6e21b51f