Config overhaul: remove default values from config file templates

[josecelano]

Parent issue: #401

The new Figment configuration implementation allows you not to define config values when you want to use the default one. For example, you can run the tracker without providing any value:

TORRUST_TRACKER_CONFIG="" cargo run

And it will use default values. For the time being, all options have a default value.

Since we don't need to specify the default values, we should refactor the default config files to remove options with default values:

$ tree share/default/config
share/default/config
├── tracker_checker.json
├── tracker.container.mysql.toml
├── tracker.container.sqlite3.toml
├── tracker.development.sqlite3.toml
├── tracker.e2e.container.sqlite3.toml
└── tracker.udp.benchmarking.toml

That makes it easier to maintain them.