Make auth_keys to list keys from all users

Changed ssh.auth_keys to list keys from all users when user is not
defined. Change is backward compatible. When user is provided the
function returns keys from the user only (however not inside user as
user_keys).
Merge pull request #15211 from toddnni/feature-list-all-users-in-ssh-auth-keys

Make auth_keys to list keys from all users