CBD-1425:  use "fetch-manifest.rb" instead of "repo init"

Change-Id: Ia6f190f65a759d59330f76005d9c422f3cbd1b73
Reviewed-on: http://review.couchbase.org/40429
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>