Cookbook defines rabbit settings

When setting a coobook's default attributes from the attributes
of another cookbook, it becomes impossible to override from
a wrapper cookbook.  We just started working on wrapper cookbooks,
and attempting to override `openstack["mq"]` settings proved
unsuccessful.

Since the community actively has people looking into wrapper
cookbooks, this fix seems reasonable.

Change-Id: Ibadc4bddd1bc66638cd826f5bc6e3f6ee3a38179