Check downburst paths. Display an appropriate error message if an
executable downburst cannot be found.

Fixes: #4693
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Reviewed by: Dan Mick <dan.mick@inktank.com>
Merge branch 'wip-teuthologyfix4693-wusui'
Add changes to make teuthology suites work on vms.

Fixes: #4719
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Reviewed by: Dan Mick <dan.mick@inktank.com>