Use the released toxgen tool instead of our copy

Update the docs to explain how to use the toxgen.py
released on pypi and use it to regenerate the tox.ini
file.

Change-Id: I9e0d28f0e40aa0280132a644260d028190b10974
Alter is_locked() helper comment

The is_locked() helper method comment
was created when we used threading2 but
since we don't use it anymore we need to
adjust that comment to reflect the current
usage.

Change-Id: I06051fb2d65b8c871e916d7e64e6056ecff3afda
Merge "Use the released toxgen tool instead of our copy"
Merge "Alter is_locked() helper comment"