Code review comment for ~smoser/curtin:feature/add-xenial-tox-env

Changes I made:

a.) move py3-flake8 up to run early in 'tox' default. it runs fast and catches some errors.
b.) drop running of 'trusty-check' by default. we do not run this in our package builds any more.
c.) add xenial-py27 environment. (and make xenial-py3 share the base 'xenial' with it).
d.) we are not running pyflakes or pycodestyle/pep8 in xenial- environments.