Remove lib/dib

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
diff --git a/stack.sh b/stack.sh
index 2ac7dfa..bf9fc01 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1227,9 +1227,9 @@
     init_heat
     echo_summary "Starting Heat"
     start_heat
-    if [ "$HEAT_CREATE_TEST_IMAGE" = "True" ]; then
-        echo_summary "Building Heat functional test image"
-        build_heat_functional_test_image
+    if [ "$HEAT_BUILD_PIP_MIRROR" = "True" ]; then
+        echo_summary "Building Heat pip mirror"
+        build_heat_pip_mirror
     fi
 fi