Group role and user management should be done via group update

[mpeck]

reference #1342

Currently adding roles and users to groups requires the user to hit two endpoints: POST /v1/groups/<id>/users and POST /v1/groups/<id>/roles. Instead we should just use the group create and update endpoints. This would allow us to create/update a group with users and roles in a single request and would allow some simplification to the api.