server: add subtree match keyword argument
server: add subtree match keyword argument

Ensure only objects in the /org/openbmc namespace are
added to the mapper cache.

Change-Id: I3d6106f77bf8f49eb20719942e76620f0ee6648d

bradbishop/phosphor-objmgr,openbmc/phosphor-objmgr,bradbishop/phosphor-objmgr,bradbishop/phosphor-objmgr,openbmc/phosphor-objmgr