Add support for MongoDB secure connections using TLS

Add a `tls` field to under `security` in `mongo-rs.yaml`:

https://github.com/Graylog2/graylog-helm/blob/5ffb07240d003f9af51ca4788b115179f7dbea31/charts/graylog/templates/custom/mongo-rs.yaml#L18

## Related Links

https://github.com/mongodb/mongodb-kubernetes/blob/master/docs/mongodbcommunity/secure.md

https://github.com/mongodb/mongodb-kubernetes/blob/master/helm_chart/templates/mongodbcommunity_cr_with_tls.yaml

https://www.mongodb.com/docs/kubernetes/v1.4/tutorial/secure-internal-auth/

https://www.mongodb.com/docs/manual/tutorial/configure-x509-member-authentication/