Support passive attributes (AKA lazy attributes)

[dmoranj]

Current implementation doesn't support passive attributes. They should be supported for both bindings.

[fgalan]

https://github.com/telefonicaid/iotagent-ul/blob/master/docs/usermanual.md include this:

Current version of the agent only supports active attributes, i.e. those attributes actively reported +by the device to the agent. Passive or lazy attributes, i.e. those attributes whose value is only given upon explicit +request from the agent, are not implemented. Please check the issue +#23 for more details and updates regarding its implementation.

This should be removed once this issue gests implemented.