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

pulp_container/app/settings.py

@@ -1,3 +1,5 @@
+from django.core.exceptions import ImproperlyConfigured
+
 DRF_ACCESS_POLICY = {
     "dynaconf_merge_unique": True,
     "reusable_conditions": ["pulp_container.app.global_access_conditions"],
@@ -8,3 +10,14 @@
 
 # The number of allowed threads to sign manifests in parallel
 MAX_PARALLEL_SIGNING_TASKS = 10
+
+
+from dynaconf import DjangoDynaconf, Validator
+
+# Validate required settings
+token_auth_enabled_validator = Validator("TOKEN_AUTH_DISABLED", eq=False)
+token_server_validator = Validator("TOKEN_SERVER", ne="", when=token_auth_enabled_validator)
+token_signature_algorithm_validator = Validator("TOKEN_SIGNATURE_ALGORITHM", ne="", when=token_auth_enabled_validator)
+public_key_path_validator = Validator("PUBLIC_KEY_PATH", ne="", when=token_auth_enabled_validator)
+private_key_path_validator = Validator("PRIVATE_KEY_PATH", ne="", when=token_auth_enabled_validator)
+