Switch to CentOS as default image for fractal.
Switch to CentOS as default image for fractal.

jbeda/compute-appengine-demo-suite-python,jbeda/compute-appengine-demo-suite-python,jbeda/compute-appengine-demo-suite-python,googlearchive/compute-appengine-demo-suite-python,gdgjodhpur/compute-appengine-demo-suite-python,gdgjodhpur/compute-appengine-demo-suite-python,jonparrott/compute-appengine-demo-suite-python,jonparrott/compute-appengine-demo-suite-python,jbeda/compute-appengine-demo-suite-python,gdgjodhpur/compute-appengine-demo-suite-python,googlearchive/compute-appengine-demo-suite-python,googlearchive/compute-appengine-demo-suite-python,jonparrott/compute-appengine-demo-suite-python,googlearchive/compute-appengine-demo-suite-python,jonparrott/compute-appengine-demo-suite-python,jbeda/compute-appengine-demo-suite-python,gdgjodhpur/compute-appengine-demo-suite-python,googlearchive/compute-appengine-demo-suite-python,jonparrott/compute-appengine-demo-suite-python,gdgjodhpur/compute-appengine-demo-suite-python