LU-3565 mdt: a new param to allocate sequences

sequence controller (aka master sequence server) exports
FLDB in proc/fs/lustre/seq/ctl-*/fldb. the same entry
can be used to modify FLDB using available sequences:
lctl set_param
  seq.ctl*.fldb="[0x0000000280000400-0x0000000280000600):0:mdt"

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I2640811c23cacb00e423fcd408318cac2ac89c6f
Reviewed-on: http://review.whamcloud.com/7027
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>