LU-1255 tests: pass $DBENCH_LIB to run_dbench.sh

LU-1255 tests: pass $DBENCH_LIB to run_dbench.sh

This patch does:
1) pass the values of $DBENCH_LIB and $DBENCH_SRC to run_dbench.sh
2) redirect the output of tar directly into the $LOG file instead of
   messing up the test output
3) check the $LOGDIR directory inside gather_logs() in case
   this function was called before init_logging()
4) fix check_logdir() and check_write_access() to avoid using
   node.$(hostname -s).yml files which should not be deleted

Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: I32350bfe3481614a2e5de7d5e7c254b09bd7e774
Reviewed-on: http://review.whamcloud.com/2376
Reviewed-by: Cliff White <cliffw@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>