Cleaner MongoConfiguration for tests

This pull request fix some problems with current unit test with MongoDB:
- MongoDB temporary instances where never shutdown properly, leaving old .exe files in the temp directory forever.
- embedmongo-spring has no added value compared to de.flapdoodle.embed.mongo (it depends on it).
- @Import(MongoConfiguration.class) on top of test cases was useless because of @ComponentScan annotation on Application class.