ACK for partial installation

[arndtjonasson]

A remnant of the older code in the server is that an ack is received as soon as the ECM has received the plugin. This causes the server to provide the ack to the user through the get_ack_status service.

But this doesn't say that the app has been installed, nor that it will be successfully, so the utility is very limited. We could remove it, but on the other hand, a way to trace what is happening and where installation fails remains useful.