duplicate key value violates unique constraint on first start with wsgi workers/threads

If the database is not set and the first start of datapusher is done via wsgi with some workers/threads (as suggested by 'High availability setup') then just the first thread that tries to create the database succeeds and the rest fail with `duplicate key value violates unique constraint`.

If the number of threads and workers is reduced to 1 on first start then nothing breaks.

A possible fix for this would be to provide a command for database initialization like in CKAN