LU-13709 tests: test lfs mkdir -c without -i

LU-13709 tests: test lfs mkdir -c without -i

Almost every test with lfs mkdir -c in the test suite also
uses option -i, so lfs mkdir -c (same as -i -1, where lustre
chooses the MDTs) is poorly tested.  Add a test for that
case, sanity test_300s.

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: Iede537d52cf445c9c9a6353338670e55a11364da
Reviewed-on: https://review.whamcloud.com/39457
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Gian-Carlo DeFazio <defazio1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>