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]