HA Docs: Document RabbitMQ in Active / Active setup

This is the first commit of active / active section for RabbitMQ
in the OpenStack HA guide.

bug #1196097
Blueprint improve-high-availability-support

Change-Id: I5d0a0c6849974b62ba18d785eff393674f86b4fb
Merge "HA Docs: Document RabbitMQ in Active / Active setup"