Fix GroupsAsSubDomains option for Mesos and Marathon (#868)

* Fix GroupsAsSubDomains option for Mesos and Marathon
* Refactor reverseStringSlice function

provider/marathon_test.go

@@ -1280,3 +1280,33 @@ func TestMarathonGetBackend(t *testing.T) {
 		}
 	}
 }
+
+func TestMarathonGetSubDomain(t *testing.T) {
+	providerGroups := &Marathon{GroupsAsSubDomains: true}
+	providerNoGroups := &Marathon{GroupsAsSubDomains: false}
+
+	apps := []struct {
+		path     string
+		expected string
+		provider *Marathon
+	}{
+		{"/test", "test", providerNoGroups},
+		{"/test", "test", providerGroups},
+		{"/a/b/c/d", "d.c.b.a", providerGroups},
+		{"/b/a/d/c", "c.d.a.b", providerGroups},
+		{"/d/c/b/a", "a.b.c.d", providerGroups},
+		{"/c/d/a/b", "b.a.d.c", providerGroups},
+		{"/a/b/c/d", "a-b-c-d", providerNoGroups},
+		{"/b/a/d/c", "b-a-d-c", providerNoGroups},
+		{"/d/c/b/a", "d-c-b-a", providerNoGroups},
+		{"/c/d/a/b", "c-d-a-b", providerNoGroups},
+	}
+
+	for _, a := range apps {
+		actual := a.provider.getSubDomain(a.path)
+
+		if actual != a.expected {
+			t.Errorf("expected %q, got %q", a.expected, actual)
+		}
+	}
+}