Add integration test for environment variable substitution in the configuration

[benjben]

In https://github.com/snowplow/enrich/commit/ec6fe746b5b00a8a2c6c31920c27c3725269aab8 we introduced the possibility to use environment variables for:

• application hocon
• Iglu resolver configuration
• enrichments configuration files

It'd be great to add an automated test to assert that this substitution works.