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

modules/openstack_project/files/jenkins_job_builder/config/projects.yaml

@@ -112,13 +112,14 @@
     node: precise
     rtfd-id: 1744
     tarball-site: tarballs.openstack.org
-    env:
-        - 26sa07
-        - 27sa07
 
     jobs:
       - python-jobs
-      - gate-{name}-unittests-{env}
+      - gate-{name}-unittests-{env}:
+          env: 26sa07
+          node: centos6
+      - gate-{name}-unittests-{env}:
+          env: 27sa07
       - hook-{name}-rtfd
       - pypi-jobs