Move the _spatial design handler to ns_server

ns_server takes care of the _spatial design handler, hence it should
also be configured in the capi.ini of ns_server.

Change-Id: I672d9e24f84f4c3ec6891e3e1789d740415fd63d
Reviewed-on: http://review.couchbase.org/40416
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>