LU-10277 utils: 'lfs mkdir -i -1' pick the less full MDTs

LU-10277 utils: 'lfs mkdir -i -1' pick the less full MDTs

If 'lfs mkdir -i -1 -c count' is specified, it will 'df' first,
and then randomly pick 'count' less full MDTs as specific MDTs.

Add sanity test 413.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I2ce1720479d37b1ae397054743afae865129fee3
Reviewed-on: https://review.whamcloud.com/30598
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>