LU-4334 utils: Only set a single property for nodes

LU-4334 utils: Only set a single property for nodes

For servicenode, failnode and mgsnode only set a single property but
concatenate data.  ZFS can only set a single property for a given
name so this prevents erroneously overwritting previous entries.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Ia0316e138b0d68dcda1d14811e43db3bbed64457
Reviewed-on: http://review.whamcloud.com/11161
Tested-by: Jenkins
Reviewed-by: Isaac Huang <he.huang@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>