Since all platforms have Python 3, use the new ensure-venv element
from the dependent change to install bindep and os-testr.
Since we are no longer using pip to install anything during the
builder, this drops the dependency on pip-and-virtualenv from
nodepool-base. Avoiding this element is our long-term goal, as it's
modification to system state are problematic in a number of ways. To
maintain the status-quo, the pip-and-virtualenv element is added
explicitly to each build's element list, with a note on it's future.
The current plan for backwards compatability is to replicate the
environment pip-and-virtualenv provides in a base role/job that can be
optionally included. To test this, provide a new node type
"ubuntu-bionic-plain" that will not include the pip-and-virtualenv
element. This is put on just one provider (rax) to minimise impact.
The dependent-change (and a dib release) is required before merge so
the ensure-venv element is available.
Depends-On: https://review.opendev.org/707513
Change-Id: I85438baf5bb31790a56fe5b38327361f0a2398e9
7602343b6a