Prepend SSH keyfile to settings in fab_exists() helper

This helper will try to check file on a remote server. To do this, it needs to
have a configured key to connect via SSH.

helpers.py

@@ -97,7 +97,8 @@ def fab_exists(role, directory):
     if role == "local":
         return path.exists(directory)
     else:
-        return exists(directory)
+        with settings(key_filename=fab_ssh_key()):
+            return exists(directory)
 
 
 def fab_add_to_hosts(ip, site_hostname):