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'

Declare a durable exchange, when an exchange is already declared
in the broker.