IMPALA-837: Use '_' as prefix for insert staging directory

Although Hive ignores directories with '_' or '.' as prefixes, some
tools only ignore those beginning with '_'.

Change-Id: I499491b0cb1919c4b3a46efcc45b57ad56bfdf86
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4985
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins