Devstack getting started, update IMAGE_URLS
Devstack getting started, update IMAGE_URLS

The current image references are F17 which are very EOL and
unsupported, so instead refer to the official F20 image which
now contains heat-cfntools, and mention that in the documentation.

Change-Id: I4906fe4f0785c23b3f8815dbaa0a4727f8ed9505

NeCTAR-RC/heat,maestro-hybrid-cloud/heat,pratikmallya/heat,cryptickp/heat,pshchelo/heat,openstack/heat,dragorosson/heat,gonzolino/heat,noironetworks/heat,dims/heat,noironetworks/heat,rdo-management/heat,cwolferh/heat-scratch,openstack/heat,srznew/heat,rh-s/heat,rh-s/heat,takeshineshiro/heat,gonzolino/heat,redhat-openstack/heat,dims/heat,miguelgrinberg/heat,miguelgrinberg/heat,takeshineshiro/heat,steveb/heat,NeCTAR-RC/heat,rdo-management/heat,pshchelo/heat,steveb/heat,maestro-hybrid-cloud/heat,redhat-openstack/heat,jasondunsmore/heat,cwolferh/heat-scratch,jasondunsmore/heat,srznew/heat,cryptickp/heat,pratikmallya/heat,dragorosson/heat