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