init: Fail if origin not using SSH

There is an issue with posting review requests if users clone their
repositories using HTTPS instead of SSH. Review Board does not recognize
such repositories and returns and error.

This changeset makes flow init fail if the origin picked up by gitflow
is not using SSH.

Change-Id: I11393558c16d708a15f51298653515e03c957eac
Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>