[REMJMX-74] Only disable local authentication if we have a username and password not just because we have a CallbackHandler.

Also added a new 'org.jboss.remoting-jmx.excluded-sasl-mechanisms' option to list SASL mechanisms that should be excluded from the list.
[REMJMX-74] Add some tests for the scenarios which should switch off local auth and the ones that should leave it enabled.