add database clean up scripts

Currently the pywps job queue handling is not working so well. The queue gets filled and no new jobs will be excepted. As a workaround we can add scripts to clean up the job queue, like in pavics:
https://github.com/Ouranosinc/PAVICS/blob/master/birdhouse/scripts/clear-running-wps-jobs-in-db.sh

A better queue handling is in the making:
https://github.com/geopython/pywps/pull/497