[MessageHub]: Use Empty type rather than Node to detach a Node

[harlem88]

Since the only information that we need for detaching an Node is the nodeId, and it is provided by GRPC metadata, we can use the Empty type rather than Node on detach method.
It depends on #163.

[sorru94]

Due to the restrictions of the v0.6.x message hub the CI is now using v0.5.4. This version includes a bug with the reception of empty arrays.
Due to that bug part of the end-to-end tests have been disabled in #169.
When tackling this issue the message hub should be updated and the end-to-end tests should be fully enabled again.