BootLinuxSshTest: Only run the tests when explicitly requested

Currently the Avocado framework does not distinct the time spent
downloading assets vs. the time spent running a test. With big
assets (like a full VM image) the tests likely fail.

This is a limitation known by the Avocado team.
Until this issue get fixed, do not run this tests automatically.

Tests can still be run setting the AVOCADO_TIMEOUT_EXPECTED
environment variable.

Reported-by: Gerd Hoffmann <kraxel@redhat.com>
Reported-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20190918122748.2144-1-philmd@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>