Allow .teuthology.yaml to set downburst path

If .teuthology.yaml defines downburst, _get_downburst_exec()
now returns that value as the path to the downburst executable.

Also cleaned up code in create_if_vm.  ctx.downburst_conf was
never defined, so the code that handled the AttributeError
was always being executed.

Fixes: 6921

Signed-off-by: Warren Usui <warren.usui@inktank.com>
Merge pull request #255 from ceph/wip-6921-wusui

Allow .teuthology.yaml to set downburst path
Fix name parsing

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Move "no results server" warning

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Move list of exceptions to catch

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Add try_mark_run_dead()

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Use try_mark_run_dead()

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Let ResultsSerializer worry about the archive dir

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Better logging

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>