resulttool/manualexecution: Enable test case configuration option

resulttool/manualexecution: Enable test case configuration option

Current manualexecution required user to exceute all test cases defined inside a "modulename.json" file in oeqa/manual

There are cases when all test cases all not required to run for a module on specific DUT.

Enable manualexecution to have the optional feature where it will use pre-defined json format test case configuration file
where user will be able to select test cases from the "modulename.json" instead of running all of them. This will help
in reducing testing time and reporting unneccesary skip or failures.

Example pre-defined json format test case configuration file (for build-applince):

{
    "testcases" : [
        "build-appliance.build-appliance.Create_core-image-sato-sdk_using_build_appliance",
        "build-appliance.build-appliance.Build_a_image_without_error_(added_recipe)"
    ]
}

Signed-off-by: sangeeta jain <sangeeta.jain@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>