Specify the URL that points the top level tag directory. For example,
job name 'MyJob' with svn URL http://svnserver/trunk/project1 is copied to
http://svnserver/tags/last-successful/MyJob/project1 if the tag base URL is
specified as http://svnserver/tags/last-successful/${env['JOB_NAME']}. Note that hostname must
be identical with one in Subversion URL.