Add global optional config data opt-out

[philwebb]

The recently introduced optional: prefix causes problems for Spring Cloud and likely for some of our users as well. We should offer a quick way to opt-out. One idea would be a spring.config.on-import-not-found=WARN/IGNORE/FAIL property.

[philwebb]

I went with a slightly simpler boolean property in the end.

[wilkinsona]

In the interests of future-proofing things, I wonder if we should use an enum with two values rather than a boolean?

[philwebb]

spring.config.on-location-not-found WARN/IGNORE/FAIL