oeqa/runtime/ssh: increase the number of attempts

Under high load, the ssh test is hitting the amount of retries.
Increase it to 20 to avoid this issue.  This would increase the maximum
failure time from 50 seconds (5 * 10) to 100 seconds.

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit c796438e)
Signed-off-by: Steve Sakoman <steve@sakoman.com>