pick default replication type using environment variable
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 <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

ceejatec/ns_server,membase/ns_server,membase/ns_server,mschoch/ns_server,membase/ns_server,t3rm1n4l/ns_server,membase/ns_server,nimishzynga/ns_server,daverigby/ns_server,mschoch/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,ceejatec/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,mschoch/ns_server,mschoch/ns_server,nimishzynga/ns_server,ceejatec/ns_server,daverigby/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,ceejatec/ns_server,membase/ns_server,mschoch/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,membase/ns_server