LU-1187 tests: Add test_mkdir in sanity for DNE.

LU-1187 tests: Add test_mkdir in sanity for DNE.

1. Replace mkdir with test_mkdir, which will create local or
remote directory according to its testnum, i.e. choose the
mdt index by testnum % mdt_count, then create directory
by lfs mkdir.

2. Some cleanups replace dxx/fxx with $tdir/$tfile, remove
"--skip" in SKIP message.

Change-Id: I7d5396ddb50c7414f78a76c20772a06da438f4e0
Signed-off-by: Wang Di <di.wang@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/4359
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>