Custom github url property not exposed to bosh for github oauth

From slack

```
superbeeny [11:01 AM] 
@vito :disappointed: but it works now, do you have the yaml for setting the endpoint for github enterprise

vito [11:03 AM] 
@superbeeny: looks like bosh properties weren't added to expose it :confused: should've caught that

[11:03] 
just atc flags
```

Bosh properties need exposed for custom github endpoint for github oauth.
