Introduce an option --keep-going to make ability to run all planned tests and
ignore failed tests. Also with this option zdtm.py will keep image files and
output log for each failed test:

[vagrant at fedora test]$ find sergeyb-xxx -maxdepth 3
sergeyb-xxx
sergeyb-xxx/zdtm_static_bridge_ns
sergeyb-xxx/zdtm_static_bridge_ns/images
sergeyb-xxx/zdtm_static_bridge_ns/images/1
sergeyb-xxx/zdtm_static_bridge_ns/output
sergeyb-xxx/zdtm_static_bridge_ns/images.0
sergeyb-xxx/zdtm_static_bridge_ns/images.0/1
sergeyb-xxx/zdtm_static_bridge_ns/output.0
sergeyb-xxx/zdtm_static_deleted_unix_sock_h
sergeyb-xxx/zdtm_static_deleted_unix_sock_h/output
sergeyb-xxx/zdtm_static_deleted_unix_sock_h/output.0
sergeyb-xxx/zdtm_static_deleted_unix_sock_h/output.1
Signed-off-by: Sergey Bronnikov <sergeyb@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>