check whether ipv6 is open,second commit

if the OS has foribided the ipv6,then DNS.getIPs[1] will ocurr ArrayIndexOutOfBoundsException
Error Information:
    Exception in thread "main" java.lang.RuntimeException: Failed construction of Regionserver: class org.apache.hadoop.hbase.regionserver.HRegionServer
    Caused by: java.lang.ArrayIndexOutOfBoundsException: 1 
    the error also ocuur in HMaster.java
    
    this patch will make hbase running normally on operating system which close  ipv6