Change KarafTestContainer to rely directly on options and KarafPropertiesFile

Please do not merge this pull request yet.

I've created it as entry point for discussion about Pax Exam options used by Karaf test container. In few places we have commas which are simply separators for properties like boot features, repositories and so on. Because of that in many places we need to keep these "," and have values prepended by it. This causes small, but additional effort in test code.

To avoid string glue for boot features and repositories I prepared additional options:
- **KarafBootFeatureOption**
- **KarafFeatureRepositoryOption**

These options allows to extend bootFeatures and featureRepositories properties in org.apache.karaf.features.cfg file.

Additional option introduced by this commit is **LogCategoryLevelOption** which allows to specify log4j.logger.<category> property inside org.ops4j.pax.logging.cfg.

During my work on these options I've found that lots of options are handled in manual way - ie. each config option has corresponding method in KarafTestContainer. Amount of code duplicated or very similar there is terrifying. If you don't mean, and if you like the approach of unifying option handling, I would like to refactor it a little bit and then merge this pull request.