Follows the IOU pattern: the file lives in gns3server/configs/ and the
controller installs it into the user's configs directory on startup
(never overwriting a customized copy). Referenced by the documented
GNS3_IOL_STARTUP_CONFIG=iol-xe-base.txt template knob — without it a
fresh install had no file for the knob to resolve and nodes booted to
the initial configuration dialog.
Also fixes the configs directory named in the docs (~/GNS3/configs,
the configs_path server setting default — not ~/.config/GNS3/...).
