MB-6972:  set S3_PACKAGE_ROOT locally to override default:

    s3://packages.couchbase.com/releases/couchbase-server

Change-Id: I2ce992f3630b2c32636a3eb9145fc38b6d587095
Reviewed-on: http://review.couchbase.org/41152
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>