Make demonstration tide gauge

Config looks like:

```
{
    "tide_gauges": [
    {
      "slug": "liverpool-demo-tide-gauge",
      "software_version": "demo-tide-gauge",
      "configuration": {
        "location_slug": "liverpool-demo"
      }
    }
    ]
}

```

https://www.pivotaltracker.com/story/show/85003628 [Delivers #85003628]