Use pre-defined data models as output.

[Jefwillems]

We could use pre-defined standardized data models as output.

Now, the apiapi only outputs "Devices" for the IOT-registry, but we could in theory enable it to output whatever we want. To give the person managing the apiapi (the one using the admin panel) not too much of a headache, we could use datamodels from OASC/synchronicity/etc... so the user can just select the datamodel he or she would like to use.