qa/workunits/snaps: New allow_new_snaps syntax

These were probably just obscuring other failures.

Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #1190 from ceph/wip-snaptest-next

qa/workunits/snaps: New allow_new_snaps syntax