tests/vm: send proxy environment variables over ssh

tests/vm: send proxy environment variables over ssh

Packages are fetched via proxy that way, if configured on the host.
That might be required to pass firewalls, and it allows to route
package downloads through a caching proxy server.

Needs AcceptEnv setup in sshd_config on the guest side to work.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190617043858.8290-2-kraxel@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>