[LOGMGR-107] Implement LogManager.getLoggerNames()

The J.U.L. ```LogManager.getLoggerNames()``` only returns loggers that have been created. Adding the ```boolean``` to the ```LoggerNode``` allows the ```org.jboss.logmanager.LogManager.getLoggerNames()``` to have the same results.

While in J.U.L the ```LogManager.getLoggingMXBean().getLoggerNames()``` does _not_ return loggers that have been created, our implementation has always done this. Therefore I decided to leave our implementation as is for now.
This will fail if nextElement() is called without a preceding call to hasMoreElements(). Needs a call to findNext() and another null check before throwing the exception.