Instanciate the SMTP class only when needed
instead of creating it in the VaultBackend constructor: when configured (with host and port), SMTP.connect() is immediately called, which makes it mandatory to have the smtp server up and running to be able to create the VaultBackend object (which makes it hard to run properly in an elastic environment like docker or k8s).
Migrated from D8205 (view on Phabricator)