zdtm.py: updated logic for --keep-going option
zdtm.py: updated logic for --keep-going option

- option --keep-going is not mandatory if we run a single testcase
- show warning when we use --keep-going with a custom set of testcases
- ignore --keep-going when we run a custom set of testcase (exit on 1st fail)

Signed-off-by: Sergey Bronnikov <1616b17a65c724245105acd00c580303fdcc2227@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

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