While the ssh.SSHTest.test_ssh is often executed in ~ 30 seconds, it
occasionally times out.  It has also been observed to pass after 192 seconds, so
300 seconds is probably just cutting it a bit close.

That said, it is really surprising that it can be so slow.  It doesn't really do
much.  Maybe it is slow because of some aftermath of the ptest test case that is
run just before.

Signed-off-by: Esben Haabendal <esben.haabendal@huawei.com>