Make tempest's python 2.6 policy clear
Make tempest's python 2.6 policy clear

This commit adds a few sentences to the end of the python 2.6 section
in the readme to clearly articulate that there is no python 2.6 in
tempest starting with kilo, and that patches to fix python 2.6 support
will be rejected.

Change-Id: I28b4714e35b84642be47961d3f51343450504db7

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