Fix credentials in some use cases

* Enable use of additional credentials in post-commit hook externals (a [variant of JENKINS-21785](https://issues.jenkins-ci.org/browse/JENKINS-21785?focusedCommentId=196312&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-196312) that isn't due to misconfiguration).
* Enable use of additional credentials when using `SubversionSCM.DescriptorImpl.createAuthenticationProvider(AbstractProject)` with a project with Subversion as SCM (e.g. Subversion Tagging Plugin).
* Added `Level.FINE` logging to allow determining realm names even in case of error within Jenkins