Random fit algo

Implement and add unit tests for naive scheduling algorithm. First filter the agents to match the container configuration and than just randomly choose one. 

Constraints for now:
- cpu
- memory
- volumes

@peterbourgon @bernerdschaefer  @jmyounker  @soundcloud/iss  
Because this is an "internal" type never used outside of this file, and because it's just a convenience type with no methods defined on it, I think it makes more sense to remove the type definition, and use a plain `map[string]struct{}` everywhere.