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 pulp#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

@@ -1,4 +1,6 @@
 from pulpcore.plugin import PulpPluginAppConfig
+from django.conf import settings
+from django.core.exceptions import ImproperlyConfigured
 
 
 class PulpContainerPluginAppConfig(PulpPluginAppConfig):
@@ -11,3 +13,25 @@ class PulpContainerPluginAppConfig(PulpPluginAppConfig):
 
     def ready(self):
         super().ready()
+
+        if missing_settings := self.get_missing_settings():
+            raise ImproperlyConfigured(
+                f"Missing required settings for token authentication: {', '.join(missing_settings)}"
+            )
+
+    def get_missing_settings(self):
+        # Checks only needed if token auth is enabled
+        if str(getattr(settings, "TOKEN_AUTH_DISABLED", False)).lower() == "true":
+            return None
+
+        missing_settings = []
+        if getattr(settings, "TOKEN_SERVER", None) is None:
+            missing_settings.append("TOKEN_SERVER")
+        if getattr(settings, "TOKEN_SIGNATURE_ALGORITHM", None) is None:
+            missing_settings.append("TOKEN_SIGNATURE_ALGORITHM")
+        if getattr(settings, "PUBLIC_KEY_PATH", None) is None:
+            missing_settings.append("PUBLIC_KEY_PATH")
+        if getattr(settings, "PRIVATE_KEY_PATH", None) is None:
+            missing_settings.append("PRIVATE_KEY_PATH")
+
+        return missing_settings