dovetail tool: skip testcase status added

dovetail tool: skip testcase status added

JIRA: DOVETAIL-344

1) testcase status, True/False --> PASS/SKIP/FAIL,
   so skipped testcase in tempest can be figured out
   details can be seen in
   https://etherpad.opnfv.org/p/dovetail_report
2) some variable name changed more readable, such as
   passed-->testcase_passed failed--> prepare_failed
3) "dovetail report" minor amend according to 1)

Change-Id: Ibe8e50693fc474d78e7d0fb89257f34703248579
Signed-off-by: MatthewLi <matthew.lijun@huawei.com>