Don't create user and extensions when (re)starting

When restarting a container with

docker stop postgres-osm
docker start postgres-osm

the entry script executes the user and extension creation
all over again which leads to the following error:

createuser: creation of new role failed: ERROR:  role "osm" already exists

This means that once a container has been stopped it can
not be started again.

The check is very rudimentary and should be replace by actually
checking whether $OSM_USER / OSM_DB / etc. exist.

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>