Make the build reproducible

Whilst working on the Reproducible Builds effort [0], the team noticed
that diskimage-builder could not be built reproducibly as it iterates
over XML elements in a non-deterministic order when generating the
documentation. This patch sorts the elements, and is a forward of the
patch sent to the Debian BTS [1].

 [0] https://reproducible-builds.org/
 [1] https://bugs.debian.org/892020

Change-Id: I40011514a135bc50383b072ab83c7f91689bad5a

doc/lib/element_deps.py

@@ -33,7 +33,7 @@ def make_dep_list(title, deps):
     lines = []
     lines.append(title)
     lines.append("+" * len(title))
-    for dep in deps:
+    for dep in sorted(deps):
         lines.append("* :doc:`../%s/README`" % dep)
     lines.append('')  # careful to end with a blank line
     return lines