Fix bugs for API steering/{{ID}}/targets

[dpham692]

I'm submitting a ...

• bug report
• new feature / enhancement request
• improvement request (usability, performance, tech debt, etc.)
• other

Traffic Control components affected ...

• CDN in a Box
• Documentation
• Grove
• Traffic Control Client
• Traffic Monitor
• Traffic Ops
• Traffic Ops ORT
• Traffic Portal
• Traffic Router
• Traffic Stats
• Traffic Vault
• unknown

Current behavior:

User can create a steering target for non steering DS.
For example:
Do a POST /api/3.0/steering/id/targets but using other DS id besides steering.
The API will response 200 OK

{
    "alerts": [
        {
            "text": "steeringtarget was created.",
            "level": "success"
        }
    ],
    "response": {
        "deliveryService": "censored",
        "deliveryServiceId": "censored",
        "target": "censored",
        "targetId": 16,
        "type": "censored",
        "typeId": 43,
        "value": -1
    }
}

Expected / new behavior:

API should not let user create target for non steering ds.

Minimal reproduction of the problem with instructions:

Anything else:

[mitchell852]

yeah, i suppose creating a target for a non-steering ds technically makes no sense as it won't be used....however, it has no negative effects.