Simplify and unify hazelcast-code-samples

I would like to open discussion about simplifying and unifying code samples. The idea behind is to provide unified experience to users and allow QE to verify the samples in a CI job.

Suggested improvements:
* use one main class per sample and unify its name across the samples (e.g. `Main.java`), so the maven-exec-plugin can be configured in the parent POM and users can use `mvn exec:java` in each sample
* shut down the whole cluster at the end of each sample
* remove shell scripts - the maven-exec-plugin shall be used instead (lets provide `README.md` where necessary)
* use `System.out` to print what the samples do
* it will allow to introduce basic test automation in Jenkins

We should also think about slimming down the samples - i.e. find candidates for a removal. We have 214 modules with 312 main classes included now. Such amount is not easy to maintain. I would prefer to have 100 main classes at most.

Comments and suggestions for other improvements are welcome.