setup ns_server app environment too in ns_couchdb process

So that path_config functions work

Change-Id: I0dd63396051c1073b18e4bbf1a6acbdcffa94e28
Reviewed-on: http://review.couchbase.org/42429
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>