LU-8508 nodemap: improve object handling in cache saving

LU-8508 nodemap: improve object handling in cache saving

Saving cache files requires that the old cache be removed. This means
that the config object reference needs to change to point to the new
file. Previously this was done in a number of different places and
was more opaque. This patch hopefully makes it more transparent.

This patch also fixes a problem on MDTs/OSTs when creating a new
config. Previously all initial config creates would assume an MGS
context. A side-effect of the improved object handling is that a
target context is handled correctly.

Signed-off-by: Kit Westneat <kit.westneat@gmail.com>
Change-Id: Iee33a423b76f30eba849288c746e6528ecefa7c6
Reviewed-on: http://review.whamcloud.com/22004
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>