Steering services feature additions

Steering services need to be able to set order or weight for their targets. Order can be any whole integer, weight must be a whole integer equal or greater than 0.

We also need to provide the ability to add more than 2 targets per steering service.

Documentation on Steering Services
http://trafficcontrol.apache.org/docs/latest/admin/quick_howto/steering.html?highlight=steering%20service