http_frontend: Correctly update configs, etc.

Docker forbids updating configurations and secrets: to replace one, one
must either create the replacement with a new name or delete and
recreate the configuration or secret, which requires stopping any
containers that use it.

This change modifies the Docker Stack deployment to give each
configuration a name derived from a hash of its contents. This allows
running stacks to be updated while preserving idempotency.

This also changes some tasks to prevent them from unnecessarily
reporting changed status, making this role idempotent.