LU-5092 nodemap: save nodemaps to targets for caching

Modify nodemap config storage to save config to targets as well as
MGSes. This allows targets to start with the last received nodemap
configuration even if the MGS is not available. The config is
replaced by the MGS' config the next time the MGS is available.

Signed-off-by: Kit Westneat <kit.westneat@gmail.com>
Change-Id: I1c5221815618fe0265908bfd900ba55f44d1021b
Reviewed-on: http://review.whamcloud.com/17503
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>