Adds a validation utility for arguments
Merge pull request #638 from pveentjer/improvement/3.x/argument-check-utility

Improvement/3.x/argument check utility
ConfigXmlGenerator: improved javadoc
Renamed to isNotNull and added argument check tot he ValidationUtil
ListenerConfig:improved javadoc and argument validation
GroupConfig: some validation cleanup
JoinConfig: improved javadoc + validation
MulticastConfig: validation improvement
NetworkConfig: lots of work on the javadoc