This adds a configuration variable url_prefix to the dashboard that
that is prepended to all URLs so you can access the dashboard at
http://$IP:$PORT/$PREFIX/. This is necessary if you wish to use a
reverse http proxy that forwards to the dashboard under a sub-path.
Fixes: http://tracker.ceph.com/issues/20568
Signed-off-by: Nick Erdmann <n@nirf.de>