[displaymanager] Handle case where config file doesn't exist or has no key

- If the config file doesn't exist, the dictionary is empty
 - If it **does** exist, it might not have key 'default_session' in it

Either case should avoid a KeyError by using get() (or setdefault,
in this context). Subsequent use of os.path.exists() is strange,
since the value is a **group** (e.g. a dictionary) in the config
file. Just check if it exists, and then fill something in.

src/modules/displaymanager/main.py

@@ -859,7 +859,8 @@ class DMgreetd(DisplayManager):
 
         self.config_data['terminal'] = dict(vt = "next")
 
-        if not os.path.exists(self.config_data['default_session']):
+        default_session_group = self.config_data.get('default_session', None)
+        if not default_session_group:
             self.config_data['default_session'] = {}
 
         self.config_data['default_session']['user'] = self.greeter_user