hbase-8816: Add support of loading multiple tables into LoadTestTool

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.95@1510057 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/0.95' into 0.95-gd
Merge remote branch 'origin/0.95-gd' into fb-YAHOO-323-0.95-gd