Extend `barman config-update` to handle models

So far barman config-update was only able to handle server configuration.

This commit extends the command so it is now also able to handle model configuration.

Besides that we enhance the command in a couple points:

It now logs the changes applied by barman config-update to the Barman configuration sections
Some basic checks and validations are made when executing the command, e.g.:
- Basic integrity of the JSON input regarding required keys when creating or update server or model configuration
- Check if the section is what the user expects it to be, i.e., do not update a server section if the user asked to update that as a model -- and vice-versa

Unit tests updated accordingly.

References: BAR-123.

EnterpriseDB / barman