Support SSL for RabbitMQ

Follow the RabbitMQ instructions to set up SSL for rabbitmq:

https://www.rabbitmq.com/ssl.html

In application.yml, servers.yml etc...

Property `useSSL` for simple SSL (no certificate validation).

Property `sslProperties' points to a protected properties file with secrets...

````
$ cat /secret/sslprops
keyStore=file:/secret/client/keycert.p12
trustStore=file:/secret/trustStore
keyStore.passPhrase=secret
trustStore.passPhrase=secret
````

servers.yml:

````
spring:
  rabbitmq:
   addresses: 10.0.0.3:5671
   username: guest
   password: guest
   virtual_host: /
   useSSL: true
   sslProperties: file:/secret/sslprops
````
I wonder if we should wrap some of these calls to catch Exceptions (such as the NPE that could be thrown here) in order to provide a more useful error message than if those exceptions propagate.