LU-2738 mdt: add proc entry to enable lfs mkdir for non-admin

Add enable_remote_dir_gid to enable lfs mkdir for non-admin user,
1. enable_remote_dir_gid = group, only users, whose gid == group
   can create remote dir.
2. enable_remote_dir_gid = -1, all users can create remote dir.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I6605016aa89ef3e2763ca14f94a763685fc689b6
Reviewed-on: http://review.whamcloud.com/5442
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John Hammond <johnlockwoodhammond@gmail.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>