Make Traefik health checks label-configurable with Marathon.

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.

templates/marathon.tmpl

@@ -20,6 +20,11 @@
       [backends."backend{{getFrontendBackend . }}".circuitbreaker]
         expression = "{{getCircuitBreakerExpression . }}"
 {{end}}
+{{ if hasHealthCheckLabels . }}
+      [backends."backend{{getFrontendBackend . }}".healthcheck]
+        path = "{{getHealthCheckPath . }}"
+        interval = "{{getHealthCheckInterval . }}"
+{{end}}
 {{end}}
 
 [frontends]{{range .Applications}}