LU-5092 nodemap: save id maps to targets in new index file

LU-5092 nodemap: save id maps to targets in new index file

Creates a new nodemap index file on registered targets and modifies
it on changes to the nodemaps. On init, the nodemap/idmaps are
loaded into memory from the index file. Currently only the MGS
registers itself.

Signed-off-by: Kit Westneat <kit.westneat@gmail.com>
Change-Id: Ie2b84e25ecc02d5d3daebf268d2f72ffaf142758
Reviewed-on: http://review.whamcloud.com/11813
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>