Split out a swarmCAOptions struct for options that are shared between
the ca and update commands.
Change the 'no trust root' message to an error.
Add some unit tests.
Signed-off-by: Daniel Nephin <dnephin@docker.com>
flags, including cert expiry, will be ignored, so warn if a user attempts
to use `docker swarm ca --cert-expiry` or something.
Signed-off-by: Ying Li <ying.li@docker.com>
