Allow a user to disable force mode when updating credentials



Since version 1.18 we are not able to upgrade the Subversion plugin anymore, the reason why we always failed to introduce the new version is because the plugin did not let us configure the correct credentials anymore.
The credentials used where always the once hard coded in the svnkit fork of Jenkins (https://github.com/jenkinsci/svnkit/blob/master/svnkit/src/main/java/org/tmatesoft/svn/core/internal/io/dav/http/HTTPConnection.java):

```... new SVNPasswordAuthentication("qaOWQ8w3-byHudson","OGQCrcJ9-byHudson",false);```

The usage of this is enforced by a call to ```authManager.setAuthenticationForced(true)``` in ```hudson.scm.SubversionSCM.postCredential(...)```.

This code is only called to test for valid credentials from within the UI.
If ```setAuthenticationForced``` is set to false, everything works fine.


This pull request allows the user to disable the forcing when entering the SVN credentials. This information is not stored and the defaults keep the same as they where before.