Fix sqlalchemy-migrate py26/sa07 job

sqlalchemy-migrate py26sa07 job (the one that runs unittests
on "old stable" version of SQLAlchemy - 0.7.x) must be run on
centos6 nodes, because precise ones don't have python2.6.

Change-Id: I4b70540fccae6ab60050a22bbbd2b88462e755f5
Merge "Fix sqlalchemy-migrate py26/sa07 job"