Switch Fuel CI for master to python 2.7

[Aleksandra Fedorova]

Several jobs on Fuel CI use python 2.6 virtual environments.

As Fuel node migrated to CentOS 7 we don't need to test python2.6 env anymore.

For every such job we need to check if it is a duplicate for openstack infra gate - then it is safe to remove.

In other case we need to split jobs to older and new parts, so that older job triggered for old stable branches keeps using python2.6, and the job which serves master branch uses python 2.7

[Aleksandra Fedorova (bookwar)]

https://github.com/fuel-infra/jenkins-jobs/blob/master/servers/fuel-ci/nailgun_performance_tests.yaml#L80 - first candidate for splitting

[Fuel Devops McRobotson (fuel-devops-robot)]

Related fix proposed to branch: master
Change author: Anton Tcitlionok <email address hidden>
Review: https://review.fuel-infra.org/14571

[Fuel Devops McRobotson (fuel-devops-robot)]

Reviewed: https://review.fuel-infra.org/14571
Submitter: Dmitry Kaigarodsev <email address hidden>
Branch: master

Commit: 4180576ed4d5d51564322096d6ab53dd8c41cf76
Author: Anton Tcitlionok <email address hidden>
Date: Wed Dec 23 10:36:50 2015

Switch to python2.7 in Fuel CI jobs

Since Fuel migrated to CentOS 7 python2.6 is no longer used, thus jobs
which use it should be switched to python2.7.
Jobs with prefix 'old' are needed to provide python2.6 compatibility
with stable/6.1 and stable/7.0 branches.

Related-Bug: #1523514

Change-Id: Ia3bad00981fea7f26ffc74a019a69255af274a9d