MB-100 Consolidation of replication parameters grabbing.

There are only few places left where ns_config is accessed directly.
Namely:
  - xdc_rep_utils:make_options
  - xdc_rep_utils:update_options
  - xdc_rep_utils:get_trace_dump_invprob
  - xdc_rep_manager:init
  - xdc_replication:handle_call({check_tokens...

It will make easier consequent integration of new replication settings
feature.

Change-Id: Ic8dd3d41215475036e2ca1ae48bb9f2c2b26dce1
Reviewed-on: http://review.couchbase.org/28178
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>