File parsing fix

With certain job inputs, users have reported the following errors:

<pre>
Traceback (most recent call last):
  File "/usr/local/openquake/oq-engine/bin/openquake", line 512, in <module>
    main()
  File "/usr/local/openquake/oq-engine/bin/openquake", line 471, in main
    log_file, args.exports)
  File "/usr/local/openquake/oq-engine/openquake/engine/engine.py", line 629, in run_hazard
    cfg_file, getpass.getuser(), log_level, exports
  File "/usr/local/openquake/oq-engine/openquake/engine/engine.py", line 686, in haz_job_from_file
    raise RuntimeError(error_message)
RuntimeError: Job configuration is not valid. Errors: {'region': ['Must specify either `region`, `sites` or `exposure_file`.'], 'sites': ['Must specify either `region`, `sites` or `exposure_file`.']}
</pre>

The problem is that the `engine` module internals changed slightly in recent days and broke the validation.