Drop python 2.6 classifier
Drop python 2.6 classifier

The time has come to remove py26 support. We don't have ability to support it
any more.
One of the main requirements of our project `oslo.db` already removed py26
classifier ( https://review.openstack.org/#/c/246198/ ) and after new release
will come, we will have the first uncompatible library in our requirements
list.
Since most of openstack-*clients will drop support of py26 soon, there is no
reason to replace oslo.db with another library.

This change is a first step to use all features of python 27(Unfortunately, a
lot of people in the world already dropped support of py27 too and switched
to py3, but openstack is a some kind of "Slowpoke", so py27 is a primary
direction for now).

PS: all code cleanup should be done in the separate patches

Change-Id: I42b503b799cc6dd6f6c68adf409a80ced9306fc5

gluke77/rally,yeming233/rally,openstack/rally,eayunstack/rally,eayunstack/rally,gluke77/rally,amit0701/rally,openstack/rally,openstack/rally,yeming233/rally,eayunstack/rally,eonpatapon/rally,amit0701/rally,amit0701/rally,eonpatapon/rally,vganapath/rally,vganapath/rally,gluke77/rally,openstack/rally,vganapath/rally,gluke77/rally,vganapath/rally