keepalived: allow vrrp traffic on a dedicated interface

Closes #550

Signed-off-by: Dimitri Savineau <dimitri.savineau@enovance.com>
To support backward compatibility with previous releases, you should to something like:

if $keepalived_vrrp_interface {
  keepalived_vrrp_interface_real = $keepalived_vrrp_interface
} else {
  keepalived_vrrp_interface_real = $keepalived_internal_interface
}

And use keepalived_vrrp_interface_real later.