qa/workunits/restart:  Wait for backtrace attr

Backtraces are stored asynchronously even after a flush,
so checking existence might race with the setxattr.  Add
a retry loop with a sleep inbetween each attempt.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
qa/workunits/restart: Setup config properly

If we run test-backtraces.py from the command line,
we need to specify the config path properly.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
qa/workunits/restart:  Flush with fewer creates

To flush segments, we set the max_segments to 1 and
segment_size to 64K, and create a bunch of files.
500 seems to be about the right number.

Signed-off-by: Sam Lang <sam.lang@inktank.com>