Group ID omitempty

[f0x11]

type Group struct {
	ID           string         `json:"id"`
	Apps         []*Application `json:"apps"`
	Dependencies []string       `json:"dependencies"`
	Groups       []*Group       `json:"groups"`
}

// Groups is a collection of marathon application groups
type Groups struct {
	ID           string         `json:"id"`
	Apps         []*Application `json:"apps"`
	Dependencies []string       `json:"dependencies"`
	Groups       []*Group       `json:"groups"`
}

ID should by omitempty.

From https://github.com/mesosphere/marathon/blob/master/docs/docs/rest-api.md#user-content-put-v2groupsgroupid.

If ID isn't empty, marathon will add a new group into origin group.