occ config:system:set should return a different output when unchanged

Steps

1. Run occ config:system:set mail_smtpmode --value mail_smtpmode
2. Run it again

Expected result

An output with "Already set"

Actual result

Exact same output.

Version

Nextcloud 20.0.2

Context

When using tools like Ansible, one might want to know whether a value was changed or not to be able to set the changed_when clause there based stdout result.

Also needs to be applied to the other occ config:*:set commands.