Offline "step ca renew" insists on --ca-url and --root parameters #627

Subject of the issue

It is irritating that the command
step ca renew renewed.crt renewed.key --offline --ca-config ./config/ca.json requires the --ca-url and --root parameters.
The documentation at https://smallstep.com/docs/step-cli/reference/ca/renew has a sample invocation that does not work at all for me:
step ca renew --offline internal.crt internal.key

Your environment

• OS - Linux (Ubuntu 20.04)
• Version - master (84e2b87)

Steps to reproduce

• Create an offline CA with CA config at config/ca.json (not the default path)
• Issue a certificate with step ca certificate c c.crt c.key --offline --ca-config config/ca.json
• Renew that certificate with step ca renew c.crt c.key --offline --ca-config config/ca.json

Expected behaviour

The certificate can be renewed with the command line above and/or a correct command line for this command is supplied at https://smallstep.com/docs/step-cli/reference/ca/renew

Actual behaviour

The command aborts with
'step ca renew' requires the '--ca-url' flag
When a dummy URL is provided, the command
step ca renew c.crt c.key --offline --ca-config config/ca.json --ca-url https://dummy.url

complains:

open /home/user/.step/certs/root_ca.crt failed: no such file or directory
The "root" property in config/ca.json is not honored.

When giving the --root and --ca-url options, the certificate is renewed as expected.

Additional context

Add any other context about the problem here.