Make debezium/example-mongodb container runnable without mongo-initiator [DBZ-2848]

Migrated from DBZ-2848

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