add --ssh option for use where https access to github is not available

Hi Giulio,
as you may recall, inside the .cms network we do not have direct access to github, and the sysadmins decided not to allow (https) access through the centrally provided proxy.
We can work around this limitation tunnelling ssh connections through the headnodes, but git-cms-addpkg uses https for the official repository.

This patch adds a --ssh option to git-cms-addpkg to allow reaching the official repository via ssh, instead os https.