API Updates for v1.1-dev (minor tweaks and additions, 3/3)

[andrewbonney]

These changes cover:

• Add a ‘controls’ attribute to Devices, permitting control endpoint exposure in the same style as Node ‘services’
• Add a device_id keys to Flow attributes, enabling Nodes which do not act as a Source (e.g. encoders, decoders, scalers)
• Permit ‘null’ flow_id against a Sender in order to allow advertisement before internal device routing has occurred

A corresponding discussion is active as part of the AMWA Incubator Basecamp

[andrewbonney]

Merging following no major issues called out on Basecamp. Full testing to be carried out once reference registry implementation updated.