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
Merge "Devstack getting started, update IMAGE_URLS"