migrate Variables.py to Variables.robot

Current integration/test: csit/variables/Variable.py supports multiple variable names which end up holding the same value. Typically, a generic (or newer) variable is set, and more specific (or older) variables get its value, for example CONTROLLER is set to value of ODL_SYSTEM_IP.

Robot supports setting variables on pybot invocation using -v option. This overrides values in Variables.py or in any Variables table in robot files. So if a user has a suite which uses ODL_SYSTEM_IP and tries to invoke it locally (not on RelEng or Sandbox), "-v ODL_SYSTEM_IP:<some_ip>" works.
But the override only happens after applying Python variable files, so if the suite uses a Resource which still refers to CONTROLLER, it will not get the <some_ip> value.

Solution is to move all variable definitions from Variables.py to Variables.robot, in which case CONTROLLER would be set properly.