Added an EventAdmin printer utility component in misc

Multiple components can be instantiated to print or log EventAdmin
events.
This simplifies the debugging of EventAdmin handlers.
Get the Framework UID property from pelix.constants

... instead of pelix.framework
EventAdmin MQTT bridge uses the pelix.misc.mqtt_client client class

... instead of the MQTT client service (soon to be removed ?)
MQTTDiscovery: Ignore loopback messages

- EDEF XML converted to ImportEndpoint beans in the __on_message method
- Empty and loopback messages are ignored
- The whole EDEF XML is sent even for removed endpoints (to allow the
loopback checking)
Removed some debug traces from MQTT RPC & Discovery