LU-12391 tests: mdsrate tests improvements

LU-12391 tests: mdsrate tests improvements

Patch improves mdsrate tests to work with
striped directories which are created if
MDSRATE_ENABLE_DNE=true.

mdsrate.c is fixed to not fail if --mdtcount
option is used and ndirs=1. Without this fix
  mdsrate --mdcount
fails as:
cannot create stripe dir: File exists
when the ranks do system(mkdir_cmd) on the
directory which was already created by the
first executed rank.

Patch sets mdt.*.enable_remote_dir_gid to "-1"
to allow mpiuser to create remote directories.

Patch makes the mdsrate based tests a bit more
verbose: mdsrate create/mknod is called with
--debug option if VERBOSE set to "true".

Test-Parameters: trivial testlist=performance-sanity
Fixes: f31c60c97328 ("LU-1187 tests: Add mntfmt/mntcount/mdtcount to mdsrate")
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-7262
Reviewed-by: Vladimir Saveliev <vlaidimir.saveliev@hpe.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Change-Id: Ieb32ad7dfad838fc9124740236889a5fe47cb901
Reviewed-on: https://review.whamcloud.com/35069
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>