Adds Marathon support.

Co-authored-by: Julien Salleyron <julien@containo.us>

provider/marathon/label.go

@@ -0,0 +1,51 @@
+package marathon
+
+import (
+	"math"
+	"strings"
+
+	"github.com/containous/traefik/provider/label"
+	"github.com/gambol99/go-marathon"
+)
+
+type configuration struct {
+	Enable   bool
+	Tags     []string
+	Marathon specificConfiguration
+}
+
+type specificConfiguration struct {
+	IPAddressIdx int
+}
+
+func (p *Provider) getConfiguration(app marathon.Application) (configuration, error) {
+	labels := stringValueMap(app.Labels)
+
+	conf := configuration{
+		Enable: p.ExposedByDefault,
+		Tags:   nil,
+		Marathon: specificConfiguration{
+			IPAddressIdx: math.MinInt32,
+		},
+	}
+
+	err := label.Decode(labels, &conf, "traefik.marathon.", "traefik.enable", "traefik.tags")
+	if err != nil {
+		return configuration{}, err
+	}
+
+	if p.FilterMarathonConstraints && app.Constraints != nil {
+		for _, constraintParts := range *app.Constraints {
+			conf.Tags = append(conf.Tags, strings.Join(constraintParts, ":"))
+		}
+	}
+
+	return conf, nil
+}
+
+func stringValueMap(mp *map[string]string) map[string]string {
+	if mp != nil {
+		return *mp
+	}
+	return make(map[string]string)
+}