The Nasqueron notifications center needs, as explained on T781,
to declare an exchange as durable.
This change allows so in AMQPBroker::setExchanteTarget.
This fixes the following AMQPProtocolChannelException:
PRECONDITION_FAILED - inequivalent arg 'durable' for exchange 'notifications' in vhost 'dev': received 'false' but current is 'true'