refactor: extract config filenames into constants

Signed-off-by: Dominik Willner <th33xitus@gmail.com>

default.kiauh.cfg

@@ -0,0 +1,18 @@
+[kiauh]
+backup_before_update: False
+
+[klipper]
+repo_url: https://github.com/Klipper3d/klipper
+branch: master
+
+[moonraker]
+repo_url: https://github.com/Arksine/moonraker
+branch: master
+
+[mainsail]
+port: 80
+unstable_releases: False
+
+[fluidd]
+port: 81
+unstable_releases: False