Dynamically set authenticator properties from YAML

We don't wanna explicitly map keys from the YAML to traitlet
config for the authentication - that's a lot of busywork for no
gain. We instead switch to using snake_case everywhere, and
dynamically set traitlet config from YAML config!

tljh/jupyterhub_config.py

@@ -4,6 +4,8 @@ JupyterHub config for the littlest jupyterhub.
 import os
 from systemdspawner import SystemdSpawner
 from tljh import user, configurer
+import yaml
+import copy
 
 INSTALL_PREFIX = os.environ.get('TLJH_INSTALL_PREFIX')
 USER_ENV_PREFIX = os.path.join(INSTALL_PREFIX, 'user')
@@ -40,4 +42,10 @@ c.SystemdSpawner.default_shell = '/bin/bash'
 # Drop the '-singleuser' suffix present in the default template
 c.SystemdSpawner.unit_name_template = 'jupyter-{USERNAME}'
 
-configurer.apply_yaml_config(os.path.join(INSTALL_PREFIX, 'config.yaml'), c)
+config_overrides_path = os.path.join(INSTALL_PREFIX, 'config.yaml')
+if os.path.exists(config_overrides_path):
+    with open(config_overrides_path) as f:
+        config_overrides = yaml.safe_load(f)
+else:
+    config_overrides = {}
+configurer.apply_config(config_overrides, c)