Update relevant auto-configurations to ConditionalOnSingleCandidate

[snicoll]

Now that we have a special support for "single candidate" in #1702 we should review our auto-config infrastructure and update conditions accordingly. Anything that configures some sort of template is directly targeted at this.

[snicoll]

It's a bit more complicated than I thought since we need to isolate such code in separate @Configuration and order them in such a way that the bean it's checking is already created. The Rabbit/JMS configuration is significantly harder to write unless we move the template config to a separate auto-configuration. Thoughts?

[snicoll]

We really should find a way to make this work. This would allow boot to back off when several instance of a given type is defined rather than blowing up like it does currently. #4678 is another occurrence of this problem.