Fixes error on OSM loader

Adds a config entry for osm2pgsql port.

osm2pgsql relies on prepared statements, and those don't play
well with transaction pooling (pgbouncer for example):

http://pgbouncer.projects.pgfoundry.org/doc/faq.html#_how_to_use_prepared_statements_with_transaction_pooling

This change provides a configurable port for osm2pgsql.