Refine commit 1e9e1b04 since it unfortunately had the side-effect of
disabling logging when a valid `log4j2.xml` file was found.

The updated code uses a similar technique but instead of accepting
'*' as a file type it now looks for `.springboot` (which it finds
because we ship a `log4j2.springboot` file in out JAR.

Rather than exiting Log4J2's `ConfigurationFactory.Factory`
`getConfiguration` method early, our `SpringBootConfigurationFactory`
is now only triggered on the last call to `getConfiguration`.

Closes gh-4809