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.
- This will help split stuff in smaller, better tested packages
- This moves some stuff like the traefik command to package `cmd`
Signed-off-by: Vincent Demeester <vincent@sbr.pm>