Allow install only of Borg and Borgmatic

[LoniasGR]

This PR aims to solve https://github.com/borgbase/ansible-role-borgbackup/issues/152 by adding when checks on relevant places. In order to only install borg & bormatic, the following would be set

    borg_repository: "" # Needs to be set to something, even empty
    borgmatic_config_name: "" # Disables borgmatic configuration
    borg_ssh_key_file_path: "" # Disables ssh key creation
    borgmatic_timer: "" # Disables timers

I tried to use existing variables, but any other suggestion is welcome.

Use case that this problem solves:

I have an existing borg/borgmatic repository server where I want to upgrade these installations without adding a new repository. It also has existing repository configurations to be able to restore the data inside the repository server, both for verification and for easier access from colleagues.