Enable device MQTT subtopics subscriptions.

[gdimitropoulos-sotec]

This issue is part of the adaption process in order to qualify Hono as a replacement for Google IoT Core customers.

At the moment when a device subscribes to a command topic, the MQTT adapter validates and checks if the topic ends with ‘#’ and also if the total number of topic levels is equal to 5.

Currently allowed subscription topic formats are:

1. command/DEFAULT_TENANT/4711/req/# unauthenticated device
2. command///req/# - authenticated device
3. command//+/req/# - authenticated gateway
4. c///q/# - authenticated device using short names

We would like that our devices are able to subscribe and receive messages only for a specific subtopic. That means without using wildcards '#' at the end and also the number of topic levels should be increased to 6.
Examples: command/myTenant/myDevice/req/+/test_subfolder, command/myTenant/myDevice/req//test_subfolder