Add support for keepalived authentication

In some circonstances, if an existing vrrp instance is on the network it
will try to join our vrrp instance, thus adding an authentication method
ensures that we won't face this situation.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
When I looked at the code some undef params didn't have that description, that's why I put it that way...