1. Add keyword Import Neuron API resource in common.resource, so that
when robotframework command line set:
1.1 `--variable neuron_api:http` will test using http api (default)
1.2 `--variable neuron_api:mqtt` will test using mqtt api
2. Add keyword Skip If Not Http API in common.resource, so that we
could run tests only meaningful for http.
3. Update test suites accordingly
1. Refactor group config api tests to use keywords from api resources
2. Refactor modbus tcp tests to use keywords from api resources
3. Refactor node api tests to use keywords from api resources
4. Refactor opcua tests to use keywords from api resources
5. Refactor tag api tests to use keywords from api resources