Parse env vars in properties

Like my [other PR](https://github.com/snowplow/snowplow/pull/996), this one is a follow up on the kinesis to redshift storage adapter.

This PR adds a little bit of logic (mostly sourced from SO) to parse ENV vars in *.properties files similarly to scala & sbt, allowing lines like:

```
redshiftEndpoint = ${REDSHIFT_ENDPOINT}
redshiftUsername = ${REDSHIFT_USERNAME}
redshiftPassword = ${REDSHIFT_PASSWORD}
redshiftURL = ${REDSHIFT_URL}
```

in say, `RedshiftBasic.properties`. That way, people can feel better about committing these files to source control (a requirement, for example, for deploying on heroku), since they won't need to expose any secrets.