Add task-args(from yaml file) candidates in /api/v2/yardstick/testcases API

Add task-args(from yaml file) candidates in /api/v2/yardstick/testcases API

JIRA: YARDSTICK-766

We have the demand to get all task-args candidates in test case yaml
file,
so that user get easily change it.
The response will like:
{
    'status': 1,
    'result': {
        'testcase': case content,
        'args': {
            'image': {'description': '', 'type': 'String'}
        }
    }
}

In this patch, I add jinja2schema 0.1.4 in requirements.txt.

Change-Id: I450082402370add5ab29090286f026fe3cc8c36e
Signed-off-by: chenjiankun <chenjiankun1@huawei.com>