MB-10793: Verify if couchdb.plt exists before copying.

Change-Id: I256c460db1465454173cc7f59a3f574b61de001f
Reviewed-on: http://review.couchbase.org/35639
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Reviewed-by: Eric Limare <eric.limare@couchbase.com>
Tested-by: Eric Limare <eric.limare@couchbase.com>