pick default replication type using environment variable

If COUCHBASE_REPL_TYPE=tap then tap is picked, if
COUCHBASE_REPL_TYPE=upr then upr is picked. Otherwise default takes
place which is tap at this time.

Change-Id: If1a24a43dd166add5198f28e328eccd153767637
Reviewed-on: http://review.couchbase.org/34392
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>