Fix for JENKINS-4043

I have applied the patch from JENKINS-4043.
The patch makes sure that existing configurations still get the /viewvc/ suffix appended.
When configuring the plugin on a job for the first time, the /viewvc/ will not be added anymore to the URL, instead the final URL will be concatenated like this: URL + "/" + location + <params>. For older ViewVC configs the URL is concatenated like this: URL + "/viewvc/" + location + <params>