Bump rabbitmq version to 3.7.7 for bionic
Bump rabbitmq version to 3.7.7 for bionic

The erlang package for bionic is 1:21.0-1 which requires rabbitmq
3.7.7.

Change-Id: Ia5f1bff278ca252a6d3228bcf94723ed2e109c68

openstack/openstack-ansible-rabbitmq_server,openstack/openstack-ansible-rabbitmq_server