tests: allow to specify venvs via $TOXENV
tests: allow to specify venvs via $TOXENV

Previously, run_tests.sh was hardcoded to run fuel-web tests on py26.
Allow to pass the correct env list via $TOXENV environment variable.
Default to "py26" for backwards compatibility.

Needed to run py27 jobs on CI.

Change-Id: I3cfe5622bd0338f9a9a41ec6b6f6ac2d3c522009

stackforge/fuel-web,huntxu/fuel-web,eayunstack/fuel-web,SmartInfrastructures/fuel-web-dev,huntxu/fuel-web,huntxu/fuel-web,huntxu/fuel-web,SmartInfrastructures/fuel-web-dev,eayunstack/fuel-web,SmartInfrastructures/fuel-web-dev,stackforge/fuel-web,eayunstack/fuel-web,SmartInfrastructures/fuel-web-dev,SmartInfrastructures/fuel-web-dev,eayunstack/fuel-web,stackforge/fuel-web,huntxu/fuel-web,eayunstack/fuel-web