Re-opening #446 which was automatically closed.

I successfully added private git repository using poetry add --git ssh://git@gitlab_url/username/repository_name.git package_name. Then I ran poetry update, but it hanged forever on cloning repository (there: https://github.com/sdispater/poetry/blob/master/poetry/vcs/git.py#L99). The problem is that I have to authenticate using my ssh key for cloning repository, git asks for the passphrase, but I cannot enter it.