Unify sanity.robot and VOLTHA_PODTests.robot

Currently the voltha-system-tests repo has two separate test suites: sanity.robot and VOLTHA_PODTests.robot. The former works exclusively with BBSim and the latter runs on physical PODs. Unify these into a single framework that works for both.

The approach is to use VOLTHA_PODTests.robot to test with BBSim. This involves (1) creating a pod config file to use with VOLTHA + BBSim as deployed by kind-voltha; (2) specifying the setup (bbsim|physical) when running the tests; (3) putting conditionals in the tests to not execute steps that are not appropriate for a specific setup.