Allow override of Licensify environment

As many of us do not run Licensify in our development environments, this will allow us to point apps at our preview Licensify build so we can still work with licences on dev.

You can point Licensify at preview by starting the stack with 
```
LICENCE_ENV=preview LICENCE_USER=<basic auth username> LICENCE_PASSWORD=<basic auth password>
```

Whilst Licensify is the only app which I've needed to use this for, there could be a use-case here for making this work across all the adapters - eg if there was a need to use preview Imminence data rather than a local copy.