add HBASE_DISABLE_HADOOP_CLASSPATH_LOOKUP switch to hbase-env.sh

hi,all

      I got a problem while I using hbase3.0.0-snapshot and hadoop 2.7.5 to build a hbase cluster,the problem is  hbase using javax.servlet-api-3.1.0-jar witch is conflict by servlet-api-2.5.jar that in

hadoop lib path, I run into hbase file and got config HBASE_DISABLE_HADOOP_CLASSPATH_LOOKUP set to false by default, this config decide whether or not include Hadoop lib to hbase class path，so the question is why we set this config to false?can we set it to true and exclude the Hadoop lib by default?