Homogenous way of setting poll commands in provisioning API

Currently, the mechanism to activate poll commands depends on the particular implementation of the IOTA using the library, as told in the README.md:

The distinction between push and poll commands will be made based on the presence of a polling flag in the device provisioning data. The details on how this flag is derived for provisioning data would depend on the particular IOT Agent implementation using this libray (in other words, there isn't any standard way of doing so). The default option (with the flag with value false or not present) is to use push commands (as they were the only ones available until the latest versions).

It would be great to have an homogenous mechanism for all agents, part of the provisioning API provided by the library. For example, a mode field for commands to state either poll or push.

Of course, the fragment of documentation above should be adjusted if this issue gets finally implemented.

Start with the provisioning API and the push/poll command discussion in README.md. Determine how provisioning data currently carries the polling flag across IOT Agent implementations. Done means a homogeneous command mode for poll or push is supported by the API and the README description is updated accordingly.