add builds for "titotest" images

With these builds plus https://github.com/dgoodwin/tito/pull/106,
I can run the full test suite for https://github.com/dgoodwin/tito
against Centos 5.9, Centos 6.4, and Fedora 20.

    git clone https://github.com/jumanjiman/tito.git
    cd tito
    sudo docker run -rm -i -t -v $PWD:/home/sandbox $image ./runtests.py \
      | tee /tmp/runtests.out ; tail -1 /tmp/runtests.out | grep OK

Without PR 106, just add `-e USER=nobody` to the above command.