feat: ship an iol-xe-base.txt base config for IOL Docker nodes

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/...).

tests/controller/test_controller.py

@@ -462,6 +462,9 @@ async def test_install_base_configs(controller, config, tmpdir):
 
     await controller._install_base_configs()
     assert os.path.exists(str(tmpdir / 'iou_l3_base_startup-config.txt'))
+    # the IOL docker base config ships with the server too (referenced by the
+    # GNS3_IOL_STARTUP_CONFIG knob in the documented template)
+    assert os.path.exists(str(tmpdir / 'iol-xe-base.txt'))
 
     # Check is the file has not been overwritten
     with open(str(tmpdir / 'iou_l2_base_startup-config.txt')) as f: