Use tripleo-core for dib-utils

We don't actually want to have separate core groups for each tripleo
project, so this shouldn't have had a new group created for it.

Per the PTL, we want to just use tripleo-core for this project.

Change-Id: I36e45c76a53edfa133881771d8ece4667fc0656e
Merge "Use tripleo-core for dib-utils"