Tox python env changed to py27 in run_tests.sh
Tox python env changed to py27 in run_tests.sh

After moving to the CentOS 7 we have no python26 env.

Change-Id: Ia9ad4438ebfd2f6c6cdc1aaebae4f52f96a333c7
Closes-Bug: #1523397

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