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
as the package name when determining releases.
* Drop the python2 shebang from manager.py in favor of specifying
the interpreter on the subprocess call. The python interpreter
version must match the python version of the OpenStack payload
due to the swift library imports.
* Enable the cPickle import in manager.py for Python 3
(C-optimized module name is _pickle), and fix 'result'
variable that is a set type but should be a dict type.
Change-Id: Ia3fdbf2020137bcf39039478ee3606717d3d6e20
Closes-Bug: #1841184
5f3735f884