Fix CHEF-3694 caused by bad resource naming

Two resources have the same name, which leads to:

WARN: Cloning resource attributes for rabbitmq_user[nova] from prior resource (CHEF-3694)
WARN: Previous rabbitmq_user[nova]: /var/chef/cache/cookbooks/rabbitmq/recipes/rabbit.rb:31:in `from_file'
WARN: Current  rabbitmq_user[nova]: /var/chef/cache/cookbooks/rabbitmq/recipes/rabbit.rb:38:in `from_file'
Merge pull request #9 from vuntz/fix-CHEF-3694

Fix CHEF-3694 caused by bad resource naming