Generic Relation for Secrets (Needs API feedback)

[AndyGauge]

Fixes: #1503

Fixes: #1503

This change replaces the Device Foreign Key with a GenericRelationship that allows extending the scope of Secrets. Currently there are test failures for the API updating devices. This work does not include extending the Secrets to any additional objects, yet. This is an example output from the api:

{
    "id": 1,
    "object": {
        "id": 1,
        "url": "http://localhost:8000/api/dcim/devices/1/",
        "name": "a",
        "display_name": "a",
        "class": "device"
    },
    "role": {
        "id": 1,
        "url": "http://localhost:8000/api/secrets/secret-roles/1/",
        "name": "Login Credentials",
        "slug": "login-credentials"
    },
    "name": "ab",
    "plaintext": "--",
    "hash": "------",
    "tags": [],
    "custom_fields": {},
    "created": "2018-08-10",
    "last_updated": "2018-08-10T18:00:23.899052Z"
}

The 'device' node is replaced with 'object' which includes the 'class' of 'device'

[AndyGauge]

I thought maybe recreating the PR with the same code would resolve the netbox database does not exist issue. Alas, not.