cbc: don't hang on bucket-flush

The http handler (in cbc) by default reads from stdin (in the case of
design doc manipulation).

Change-Id: I013a67ffe74c73dc7976e8db0000c72fa8fa62f4
Reviewed-on: http://review.couchbase.org/41947
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>