zdtm: allow --keep-going anytime when running multiple tests

Currently it's possible to continue testing despite failures only when
running zdtm.py with --all option. Let's make --keep-going option also
valid when more than one test is requested with --tests or --from.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>