LU-14992 tests: sanity/replay-vbr mkdir on MDT0

LU-14992 tests: sanity/replay-vbr mkdir on MDT0

Replace mkdir with mkdir_on_mdt0() for sanity test 133a
and relay-vbr test 7a.  These tests expect the newly
created directory is on MDT0.

Test-Parameters: trivial mdscount=2 mdtcount=4 testlist=sanity
Test-Parameters: env=SLOW=yes mdscount=2 mdtcount=4 testlist=replay-vbr
Signed-off-by: James Nunez <jnunez@whamcloud.com>
Change-Id: Icea2923a8d8d3a3aa0ddf0401f0a025480b2f6f0
Reviewed-on: https://review.whamcloud.com/44902
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Kevin Zhao <kevin.zhao@linaro.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>