Add `omitempty` to all appropriate config struct fields

[evan-bradley]

Is your feature request related to a problem? Please describe.

Right now when marshaling our config structs into YAML, they produce a large number of keys that are unset. We should get as close to only emitting fields that have been actually set as possible.

Describe the solution you'd like

Use mapstructure's omitempty suffix to omit these values from serialization.

Describe alternatives you've considered

We can add an option to confmap.Marshal that forces this regardless of whether it has been set on the struct. We may need this anyway, but it would be nice if our config structs handled this out-of-the-box.