Add diskimage-builder element.

Installs diskimage-builder from git. It's useful to have this in an
image since cloud instances themselves are often good build hosts to run
diskimage-builder (large amounts of ram for tmpfs, etc). Especially if
you have limited resources locally.

I considered adding this to the diskimage-builder repo instead of
tripleo-image-elements. If feedback suggests the other way, I'll submit
a patch for diskimage-builder instead.

Change-Id: I237ad800255d1ba927bfaa15c44dc352589b2df8