Apps: Set ussage message for all the apps

### Summary
When doing ` --help`, besied the gflags being showed, a usage message should be displayed.
This can be set up via `gflags::SetUsageMessage()` method.

### Victory condition
Add a usage message to all the apps.

### Implementation suggestion
See `maliput_query` app, it is already being used there.

Start by inspecting the maliput_query app, which already uses gflags::SetUsageMessage(), and compare its entry point with the other apps. Run each app with --help to identify which outputs lack a usage message; done means every app displays one alongside its gflags help.

Written by the indexing model from the issue text.