fix rabbitmq ssl config

The intent, per the comments in the module, is that $sensu::rabbitmq_ssl
would be set to true if you set either a certificate or a key. That var
defaults to false. Current code does not actually override this value.
When the sensu_rabbitmq_config is called, it directly references this var,
so unless you've specifically set it in your config, it's always false.
This makes it behave the way the module comments describe.
Merge pull request #268 from patrick-minted/fix_rabbitmq_ssl

fix rabbitmq ssl config