Expose details on devices marked for deletion

[davidebriani]

Astarte supports deleting devices, as described in issue #392.
However when a device is marked for deletion, this information is not exposed by Astarte APIs: e.g. the device still appear in the device list when querying Appengine for the list.

The current behavior leads to both an odd UX and external systems not being able to rely on a consistent source of truth.
By exposing whether a device is marked for deletion would allow systems that are based on Astarte (e.g. Astarte Dashboard) to distinguish it from the other devices.

[davidebriani]

Besides exposing the device details, an additional argument could be considered for the query to list devices in order to filter the list and return only the devices not marked for deletion.