When VPP port creation fails agent should notify the ML2 framework

[naveenjoy]

The VPP REST agent is only programmed to print a failure message to the console if the VPP port creation fails. The ML2 mechanism driver does a PUT of the port creation request to the agent and assumes that everything went well. This results in the creation of a "failed port" for the VM by ML2 that is not detected by the driver.

Solution:
Program the ML2 framework to detect the failure.
optionA) Use the existing ML2 messaging framework
optionB) Implement a callback mechanism

[naveenjoy]

fixed