use "rabbit_hosts" config option instead of "rabbit_host"

This allows for easy client configuration against clustered RabbitMQ
setups.  Does not break existing configs.

Change-Id: I2b180f8860a727e35d7b465253689e5e8c44eb98
Closes-Bug: 1286411
Merge "use "rabbit_hosts" config option instead of "rabbit_host""