Add AMQP configuration support

Default attributes match default carbon-cache settings.

I also incorporated your feedback to use an if-else block in the carbon.conf.erb file.

If a user upgrades to this version of the cookbook from a previous version the carbon.conf file *will not* change, therefore carbon-cache will not be restarted as a result of this change.  Unless of course enable_amqp is set to true :smile:.