Fix generating SSH key fingerprints with OpenSSH 6.8.
Created by: sstanovnik
OpenSSH 6.8 introduces a new feature that changes the default fingerprint format and algorithm used by ssh-keygen. This breaks adding new SSH keys, because GitLab expects the colon-delimited format.
The message the user sees on the Add an SSH Key screen is "Fingerprint cannot be generated", similar to #7413 (closed), but the underlying cause is different.
This change checks the OpenSSH version and explicitly specifies the previous format if needed.