Clarifying the meaning of 'instance' word throughout the code, logging and command outputs.

There is a confusion coming from word instance referring to CTA instance, Disk Buffer Instance and lately we could invent even Scheduler Backend Instance.

The command outputs are not clear on what instanceName,instance or similar refer to and usually this is inferred from the value itself.

We shall make clear what instance refers to in command outputs, logging output and the code itself.

We could redefine these terms in the config as ctaInstanceName, diskInstanceName or follow logic we started with schedulerBackendName and continue with diskBufferName and ctaInstanceName (using the word instance only once with clear definition this means production / preproduction / CI). Any other ideas that would make more sense ?

In the process, we shall make the current instanceName aka cta.instance_name appear not only in the logs of the frontend, but also be reachable and outputted via the cta-admin version command.