As described inline, deprecate the "source" install for CentOS 8.
Overwriting the packaged tools has long been a pain-point in our
images, and the best outcome is just not to play the game [1].
However, the landscape remains complicated. For example, RHEL/CentOS
8 introduces the separate "platform-python" binary, which seems like
the right tool to install platform tools like "glean" (simple-init)
with. However, platform-python doesn't have virtualenv (only the
inbuilt venv).
So that every element doesn't have to hard-code in workarounds for
these various layouts, create two new variables DIB_PYTHON_PIP and
DIB_PYTHON_VIRTUALENV to just "do the right thing". If you need is
"install a pip package" or "create a virtualenv" this should work on
all the platforms we support. If you know more specifically what you
want (e.g. must be a python3 virtualenv) then nothing stops elements
calling that directly (e.g. python3 -m virtualenv create); these are
just helper wrappers for base elements that need to be broadly
compatible.
[1] http://lists.openstack.org/pipermail/openstack-infra/2019-September/006483.html
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