test-features.sh wasn't testing anything for subnodes
so this adds a subnode feature support matrix test for
grenade. It also highlights that the grenade config
is applied to all nodes regardless of role so a TODO
is added to test_matrix.py in case someone needs to make
that per-role, per-config service override support at
some point (hopefully that never really happens and all
jobs are moved to zuulv3 by that point).