Make instack-build-images build fedora-user by default
Make instack-build-images build fedora-user by default

If we build the fedora-user image by default, then the
instack-test-overcloud script "just works". This provides a basic
sanity check for the installed overcloud.

Change-Id: Ia7aa73b7590a53810a8883af1707af0d9fa4c98d

rdo-management/instack-undercloud,eprasad/instack-undercloud,bcrochet/instack-undercloud,dpkshetty/instack-undercloud,dmsimard/instack-undercloud,dpkshetty/instack-undercloud,rdo-management/instack-undercloud,bcrochet/instack-undercloud,eprasad/instack-undercloud,dmsimard/instack-undercloud