Allow preventing retries of failed jobs

If `config.frontend.failed_job_lockout_time` is set, the failure is
cached for the specified time and the jobs can not be force-retried
during this lockout period.

Change-Id: Id7a0e02220a3d69a0b67b6628cd26eb57213558e
Ensure some configs are reasonable

Change-Id: I9c19c90984b672d669052fcba667112317920b69