For the two existing health check parameters (path and interval), we add
support for Marathon labels.
Changes in detail:
- Extend the Marathon provider and template.
- Refactor Server.loadConfig to reduce duplication.
- Refactor the healthcheck package slightly to accommodate the changes
and allow extending by future parameters.
- Update documentation.
Do not wait a full tick cycle to execute the first health check.
Additional changes:
- Make request timeout configurable (for testing purposes).
- Support synchronizing on health check goroutine termination through an
internal wait group (for testing purposes).
- Stop leaking by closing the HTTP response body.
- Extend health check logging and use WARNING level for (continuously)
failing health checks.
