qa/tasks/ceph_manager: avoid test_map_discontinuity stall with too few up osds

qa/tasks/ceph_manager: avoid test_map_discontinuity stall with too few up osds

Some tests have m=2,k=2 and this will break them.  Sometimes even if we
have 5 up osds, we end up with 4 and CRUSH gets picky, so build in a
buffer and only do this if we have 6 up.

We don't have an easy way from here to see what the min up osds for healthy
is...  basically this map discontinuity test just sucks.

Signed-off-by: Sage Weil <sage@redhat.com>