To match with the output of "ucp install --help". The "--pod-cidr" is important to let the user choose CIDR when the default pod IP range 192.168.0.0/16 has conflicts to the nodes' network.
--existing-config is a flag that uses the latest UCP config found in the
environment and does not accept a config as a flag variable. This
should help clarify that.
Signed-off-by: Kyle Squizzato <kyle.squizzato@docker.com>