dovetail tool: ensure sub_testcase_list and testcase are not None

dovetail tool: ensure sub_testcase_list and testcase are not None

JIRA: DOVETAIL-52

1) save return value of testcase.sub_testcase() in sub_testcase_list
for multiple calls later
2) ensure sub_testcase_list is not called with None value
3) ensure testcase is not called with None value
4) identify None testcase with 'Undefined' in dovetail_report.txt

Change-Id: Ifcf0c0846dc788575310f29932a4757752f1e3a3
Signed-off-by: zshi <zshi@redhat.com>