ssh publisher: add always-publish-from-master support

Allow to SSH from master, instead of hardcoding to false.
It's particularly useful when the SSH credential are only
stored on the master. Default is kept to False.

Change-Id: I365cad043d30b6b290828a24635322c7f5afc35e
Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>