LU-1203 mdt: recognize old rootsquash and nosquash_nid params

LU-1203 mdt: recognize old rootsquash and nosquash_nid params

Change mdt_process_config() to make it capable of recognizing
old "mdt.rootsquash" and "mdt.nosquash_nid" parameters.

The new parameters are "mdt.root_squash" and "mdt.nosquash_nids".

Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: I9fc71e2a997a3b0835a3adad2ccfde6d2c64ca56
Reviewed-on: http://review.whamcloud.com/2680
Reviewed-by: Liang Zhen <liang@whamcloud.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Oleg Drokin <green@whamcloud.com>