Validate settings before running Pulp instance

When token authentization is enabled, 4 additional variables have to be
set. The state of these variables is now checked, while properly
informing the user, instead of relying on exceptions raised later during
the instance's run.

closes #1550

CHANGES/1550.bugfix

@@ -0,0 +1 @@
+Pulp Container specific settings are now properly validated at startup of a Pulp instance.

pulp_container/app/__init__.py

@@ -11,3 +11,4 @@ class PulpContainerPluginAppConfig(PulpPluginAppConfig):
 
     def ready(self):
         super().ready()
+

pulp_container/app/dynaconf_hooks.py

@@ -0,0 +1,28 @@
+from dynaconf import Validator
+
+
+token_auth_disabled_validator = Validator("TOKEN_AUTH_DISABLED", eq=True)
+token_server_validator = Validator("TOKEN_SERVER", must_exist=True)
+token_signature_algorithm_validator = Validator("TOKEN_SIGNATURE_ALGORITHM", must_exist=True)
+public_key_path_validator = Validator("PUBLIC_KEY_PATH", must_exist=True)
+private_key_path_validator = Validator("PRIVATE_KEY_PATH", must_exist=True)
+
+
+def post(settings) -> dict:
+    """
+    Post load hook for Pulp settings to validate Container-specific variables.
+    """
+    container_settings_validator = token_auth_disabled_validator | (
+        token_server_validator
+        & token_signature_algorithm_validator
+        & public_key_path_validator
+        & private_key_path_validator
+    )
+    container_settings_validator.messages["combined"] = (
+        "When token authentification is enabled ('TOKEN_AUTH_DISABLED=False'), all of the "
+        "following settings variables must be set: 'TOKEN_SERVER', 'TOKEN_SIGNATURE_ALGORITHM', "
+        "'PUBLIC_KEY_PATH', 'PRIVATE_KEY_PATH'. Please check your Pulp config file."
+    )
+    container_settings_validator.validate(settings)
+
+    return {}