oeqa/ssh: Further improve process exit handling

oeqa/ssh: Further improve process exit handling

It looks like there were further cases where orphaned processes may be left
behind since the .kill() calls may be unsuccessful if the process terminated
due to the terminate or through normal exit. In that situation .wait()
wouldn't have been called.

Further tweak the exit code paths to ensure .wait() is called to update the
returncode value before returning in all cases.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 0a0a1731e38edfa72a141e8fd8f2de52be562e94)
Signed-off-by: Steve Sakoman <steve@sakoman.com>