conversion-service: check whether the DB is up

Created by: das-g

The ping-based check at https://github.com/geometalab/osmaxx-postgis-conversion/blob/ec00ce314b960a860b3042d7d1551283002b3649/conversion_service/entrypoint.sh#L10 stopped working with docker 1.5 and was/will be replaced or augmented with a hardcoded wait.

• get rid of the hardcoded wait again and check in the loop weather the DB is really up. (E.g. by connecting with psql)