server: ensure check_config is called during app instantiation
check_config converts the "stored" configuration (with toplevel keys for
most services) into a config suitable for the get_vault
function.
Ensure all tests use the same codepath to make sure it is properly exercized, and that the vault backend object can be properly initialized in all cases.
Also contains a couple of refactoring commits to unconfuse the configuration generation via fixtures.
Supersedes !144 (closed)
Test Plan
pytested + tested that the addition of check_config makes the staging deployment happy
Migrated from D7296 (view on Phabricator)