DMF health check (#577)

* DMF support PING message for health checks.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Device simulator supports PING.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Code cleanup.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Revert accidental checkin.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Fix tests.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Simplify API.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Remove simulator dead letter.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Remove dead code. 

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Reduce code.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

* Add message for one more error case.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

hawkbit-dmf/hawkbit-dmf-api/src/main/java/org/eclipse/hawkbit/dmf/amqp/api/MessageType.java

@@ -15,7 +15,7 @@ package org.eclipse.hawkbit.dmf.amqp.api;
 public enum MessageType {
 
     /**
-     * The event type.
+     * The event type related to interaction with a thing.
      */
     EVENT,
 
@@ -29,4 +29,14 @@ public enum MessageType {
      */
     THING_DELETED,
 
+    /**
+     * DMF receiver health check type.
+     */
+    PING,
+
+    /**
+     * DMF receiver health check reponse type.
+     */
+    PING_RESPONSE;
+
 }