Make overwrite behavior configurable in config.ini

By default, map-regiters overwrite old mappings.  The behavior is
currently configurable on the CLI.  Make it configurable from the
opendaylight configuration/config.ini global configuration file.

Change-Id: Idccb391154d27d8e25638c1a809088ff3d177f86
Signed-off-by: Lorand Jakab <lojakab@cisco.com>