Cleanup default config settings.

[tublitzed]

With this change to config-rs, a bug has been fixed that was preventing us from using the Default implementation for the Settings struct. We can remove most or all of the calls to set_default in Settings::with_env_and_config_file in favor of the defaults set in the Default implementation.

[pjenvey]

This is a pretty easy cleanup to accomplish, by:

• adding #[serde(default)] to Settings, which makes the Default impl be utilized which lets us..
• kill the many set_default calls that duplicate those defaults in Settings::with_env_and_config_file
• additionally, when removing the set_default calls, spot check their values -- ensure the Default implementation's values match those (just in case any have gotten out of sync over time)

As an example see how this was done for contile

[ethowitz]

JIRA

[ethowitz]

This will be completed as part of the crate reorganization work