Improve test coverage in NEC plugin

blueprint nec-plugin-test-coverage

This commit adds the following changes in unit tests.
- Unit tests to check communications with the controller in NEC plugin
- Unit tests for nec plugin agent
- Improve coverage of OFC drivers
- Mock the client to talk with the external controller in db.

Change-Id: I2b9ab0480da312ea5e804ca04b543cc144b7e545
Merge "Improve test coverage in NEC plugin"