Add settings support for provisionwatchers

[tonyespy]

Dynamically create provisionwatcher instances based on configuration settings so that device services can implement dynamic discovery.

For more details see the Device Services functional requirements document.

Estimated effort: 3d

[jduranf]

If it's fine, I will work on it

[cloudxxx8]

Sure, thank you very much

[tonyespy]

Let's discuss this. I'd prefer for someone to review the original design and document it before we re-implement in Go. For one, I think requiring an endpoint for discovery is overkill, as it'd require the use of the support scheduler. Instead, it'd be simple to add a scan interval to the SDK, so that the DS could be configured to use a sensible interval and the SDK itself would schedule the required polling.

[jduranf]

If I'm not wrong, the idea of this issue it to implement the provision system, the cache system and the callback system.
After that, in #29 should be implemented the discover logic, which can be done implementing an endpoint or/and a scan interval to the SDK (for flexibility, maybe should be better to have both options)

[cloudxxx8]

@jduranf agree, they should be implemented together

[tonyespy]

We discussed during today's WG meeting and it was decided to defer SDK-based dynamic device discovery to Fuji. It was felt that someone should review the original design first, and expand the design to also cover black-listing, before we implement anything.

It also should be noted that a DS can implement dynamic device discovery on its own with both C and Go SDKs.