diskimage-builder is a utility rather than a service, and is already
installed in devstack via pip when required.
lib/dib was created to allow an image to be created during a devstack
run for the heat functional tests, however this approach is no longer
being taken and there are no other known uses for lib/dib.
This change removes lib/dib and moves the pip mirror building to
lib/heat so that snapshot pip packages of the heat agent projects can
be made available to servers which the heat functional tests boot.
This also removes tripleo-image-elements, which has never
been utilised, and since images won't be created
during heat functional test runs it is no longer required.
Change-Id: Ic77f841437ea23c0645d3a34d9dd6bfd1ee28714
249e36dec6