JGRP-2254 Use default DNS provider when dns_address is null
JGRP-2254 Use default DNS provider when dns_address is null

pferraro/JGroups,Sanne/JGroups,pferraro/JGroups,pferraro/JGroups,Sanne/JGroups,belaban/JGroups,Sanne/JGroups,rhusar/JGroups,belaban/JGroups,belaban/JGroups,rhusar/JGroups,rhusar/JGroups