Set prefetch count to 64 for Rabbit driver

we are ditching the downstream patch to oslo.messaging that silently
overwrote the default for this option from 0 to 64, and moving such
override explicitly to the configuration management.

Initial comment on why we are doing it is included in the rabbit section
template.

Change-Id: Id47d2b3128b06fe9ea124de2a84ff756184d7ba7
1 file changed