Support params to individual test cases

[kannangce]

At the moment we will not be able to pass parameters for individual test apis and the parameters are available only at the level of test scenarios.

This has a limitation when we have to reuse a single api definition with different parameters under a given scenario. For ex, when testing a delete user scenario, get_user has to expect 200 response code before deletion and 404 after deletion of the user. To enhance the support, there were a couple of discussion that happened and we planned to go ahead with the below approach,

• Create a new sheet(scenario_test_mapping).
• This sheet will contain the following columns,
  • scenario_name
  • test_api
  • test_api_param
• The existing test_apis column in Test_scenarios will continue to exists for simple use cases and the users will use the mapping only if they have to control params at individual test level.
• If a scenario contains tests in both Test_Scenarios and scenario_test_mapping, the one in scenario_test_mapping takes precedence.
• The rationale has to be reflected in the sheet structure.

[bharathyes]

Related to #23