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>
Merge pull request #657 from enovance/keepalived-auth

Add support for keepalived authentication