Add DeDot in add_docker_metadata processor (elastic#9505)

* Never default to a qualifier when none of them are set. (elastic#9148)

Remove default version qualifier and rename the environment variable to set it from `BEAT_VERSION_QUALIFIER` to `VERSION_QUALIFIER` this will align with other parts of the stack.

**Tested with filebeat.**
```
 ❯ ./filebeat version                                                                                                                                                                                                                                                                                                                                          [08:39:01]
filebeat version 7.0.0 (amd64), libbeat 7.0.0 [0a0c267 built 2018-11-19 13:38:15 +0000 UTC]
```

**Without the patch**
```
 ❯ ./filebeat version                                                                                                                                                                                                                                                                                                                                          [08:40:07]
filebeat version 7.0.0-alpha1 (amd64), libbeat 7.0.0-alpha1 [b007837 built 2018-11-19 13:39:59 +0000 UTC]
```

Fixes: elastic#8384

* Add DeDot in add_docker_metadata processor

* Add dedot into config and default to be false

* Update changelog and documentation

* Add documentation into processors-using.asciidoc

* Run mage fmt update under x-pack filebeat

* Run mage fmt update on x-pack metricbeat

* Run update again

* Run make update fmt from top level dir

* Remove mistakes from rebase

* Remove repeated doc

journalbeat.reference.yml

@@ -225,6 +225,7 @@ journalbeat.inputs:
 #    match_source_index: 4
 #    match_short_id: false
 #    cleanup_timeout: 60
+#    labels.dedot: false
 #    # To connect to Docker over TLS you must specify a client and CA certificate.
 #    #ssl:
 #    #  certificate_authority: "/etc/pki/root/ca.pem"