test: Fix `flynn cluster add` command
test: Fix `flynn cluster add` command

Config.Domain was removed in 1c4d4be.

Signed-off-by: Lewis Marshall <748e1641a368164906d4a0c0e3965345453dcc93@lmars.net>

benbjohnson/flynn,lmars/flynn,philiplb/flynn,philiplb/flynn,lmars/flynn,benbjohnson/flynn,flynn/flynn,benbjohnson/flynn,lmars/flynn,benbjohnson/flynn,benbjohnson/flynn,flynn/flynn,flynn/flynn,lmars/flynn,flynn/flynn,philiplb/flynn,lmars/flynn,philiplb/flynn,philiplb/flynn,flynn/flynn,benbjohnson/flynn