Investigate P4 Provider implementation

Support P4 enabled devices. This should be implemented as a provider. There are several things that should be thought of when sketching out the provider:

1. What abstractions are sufficient for the core to operate on a p4 device.
2. P4 enables devices to add capabilities at runtime, how should these be exposed to the application.
3. Should ONOS compile the p4 program itself, so it is not victim to the p4 program deployed on the switch?

This is work which may be done in collaboration with an external contributor.