ganglia: graphdir must be an absolute path

./graph.d would yields an error in graph.php. It must be defined as an
absolute path. Since conf.php and graph.d are in the same directory, we
can use the PHP magic keyword __DIR__.

Bug: 63362
Change-Id: I51429a0da94cd6c3ab6e80c96cd639ec8fbfab46
Merge "ganglia: graphdir must be an absolute path" into production