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/...).

gns3server/configs/iol-xe-base.txt

@@ -0,0 +1,16 @@
+!
+service timestamps debug datetime msec
+service timestamps log datetime msec
+!
+hostname %h
+!
+no ip domain lookup
+!
+ip tcp synwait-time 5
+!
+line con 0
+ exec-timeout 0 0
+ privilege level 15
+ logging synchronous
+!
+end