Make debezium/example-mongodb container runnable without mongo-initiator

Currently we are using a simple copy of an empty Mongo DB 3.6 as debezium/example-mongodb image. The `init-inventory.sh` file included is not in use (might only be used for downstream tests). The initialization over `mongo-initiator` container is unneccessary and can be removed so we ship an ad-hoc example image that comes properly setup for Debezium use-cases.

The Debezium UI does the initialization like this with a standard Mongo DB container:
https://github.com/debezium/debezium-ui/blob/master/docker-compose.yml#L48-L61