Migrate to Spring Boot - Refactor application profiles

[goekay]

Currently, we have https://github.com/steve-community/steve/tree/master/src/main/resources/config with different folders as profiles and some (once again) some pom.xml ceremony to select the profile and resource filtering.

Considering Spring Boot expects multiple profile-related files to co-exist with different name postfixes (such as application-dev.properties and application-prod.properties etc.), we should switch to this convention.