Make Device attribute optional for roaming and reachability subscriptions

[akoshunyadi]

Problem description
As stated in the Camara design guideline when using 3-legged access tokens and the device can be uniquely identified by the token, the Device identifier in the API request can be omitted.

Possible evolution
Make the attribute Device optional in the subscription creation request for both device-roaming-status-subscriptions and device-reachability-status-subscriptions.

Alternative solution

Additional context

[akoshunyadi]

As decided in the last sub-project status call, the issue can be closed, see previous comments in the corresponding PR #191