This drops xenial testing of our base roles from system-config. This
Xenial cleanup change is probably one of the more risky ones while we
still have Xenial nodes in the real world. The reason for that is the
base roles are applied in a dedicated job and if that job fails a lot of
other jobs will refuse to run. Since we're no longer testing xenial
against those roles once this changes merges we may run into unexpected
failures.
That said we don't have much way around this if we're going to drop
xenial test nodes. Perhaps we should hold off on merging this change
until we're closer to removing xenial from nodepool though.
Change-Id: I25bddd7455d79c2d96c36d4fb0dd1267954d6d17
97a2ef27f5