distro: use index instead of image name in build step names
distro: use index instead of image name in build step names

buildbot limits step names to 50 characters, so replace
the machine and image names with just the step number.
The names still appear in the description of the step.

Signed-off-by: Matt Madison <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@madison.systems>

madisongh/autobuilder