Test: cts-lab: Drop -g/--dsh-group option

Now that the cts script is gone, nothing uses this option. It's simpler
to pass nodes directly. Perhaps someday we'll have a cts-lab config file
that maps cluster names to node lists, but as of right now that doesn't
seem worth it.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>