collectd: make amqp plugin a selectable feature

The version of rabbitmq-c that collectd is compatible with, is older
than currently available 0.5.0. This results in build errors
triggerred by API deprecation at rabbitmq-c side. Make amqp
plugin a selectable feature, disabled by default.

Signed-off-by: Maciej Borzecki <maciej.borzecki@open-rnd.pl>
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>