Allow to override default ssh transport_cb

W/o this patch it is not possible to have a third party ssh transport_cb if GIT_SSH is disabled or a third party transport_cb which has a higher priority than the default one.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Added some tests

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Merge pull request #2490 from csware/ssh-wintunnel

Allow to override default ssh transport_cb - in order to allow third party ssh transports