MB-8881 set COUCHBASE_TOP variable for Darwin installation

Change-Id: I7682c8ea8cc9172a50440381e284d38a3f8ba671
Reviewed-on: http://review.couchbase.org/28380
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>