support configing buildserver VM per-build with sudo=

This adds the 'sudo' build field, which is just a script that is run as
root.  For more info, see the issue that this closes:

refs #318
closes #317
2 jobs for sudo in 33 minutes and 8 seconds (queued for 12 seconds)
Status Job ID Name Coverage
  Test
passed #21356838
metadata_v0

00:02:55

passed #21356830
test

00:33:08