Ensure tests run under python2.7

Ensure tests run under python2.7

tox uses whatever python version that tox ran with as the default python
version for environments (py27,py3x, etc are given a more specific
default).

Debian's python-tox (for example) uses python3, but we require python2.7
- and a naive `apt-get install python-tox; tox -epep8` fails with
py3-related errors on Debian.

This change explicitly sets `basepython = python2.7` in several testing
environments that would otherwise have used the default basepython.

Change-Id: I377ac6d72dec5b85c105d8a1a74f6974efb84dcf