Ensure taskflow py26 environments are using centos6 node

It appears that python2.6 is only on centos6 nodes and
otherwise is not on other node types. To ensure that our
tox venv builds correctly for 2.6 make sure we specify
that exact node type.

Change-Id: Idc4c1efeb790bba198ed17f640945d861c18a9fc
Merge "Ensure taskflow py26 environments are using centos6 node"