Automatically fill in SSH known hosts for pull mirrors

ZD: https://gitlab.zendesk.com/agent/tickets/112398

I think we had an issue where the user created a pull mirror and didn't bother to click on the "Detect host keys" button. the ssh_known_hosts field was empty. It wasn't obvious that this was an issue.

@nick.thomas Is the user is required to click the "Detect host keys" button? Is there a reason it doesn't get automatically populated? I do see in EE::Projects::MirrorsController:

         # If the known hosts data is being set, store details about who and when
          if import_data[:ssh_known_hosts].present?
            import_data[:ssh_known_hosts_verified_at] = Time.now
            import_data[:ssh_known_hosts_verified_by_id] = current_user.id
          end