PR #3495 modifies saved configuration on non-cluster devices

[robertcheramy]

@systeembeheerder writes in https://github.com/ytti/oxidized/pull/3495#issuecomment-2865477260:

Other case outputs error. but no problem. I do think that is a problem. After this update i get a diff on ALL my SRX devices:

+# error: Chassis cluster is not enabled.

An successful config backup shouldn't add errors imho

Originally posted by @systeembeheerder in https://github.com/ytti/oxidized/issues/3495#issuecomment-2865477260

[systeembeheerder]

A solution would be something like:

  when /^srx/
    out << cmd('show chassis cluster status') do |cfg|
      cfg.include?('error: Chassis cluster is not enabled.') ? '' : comment(cfg)
    end