tftp: listen to both IPv4 and IPv6

tftp should listen to both IPv4 and IPv6 to enable cases where
the ironic deploy network is on a IPv6 network.

Change-Id: Ib0f28b0b414c2d261e715791b98faf84183c7f40
Partial-bug: #1628234

manifests/pxe.pp

@@ -100,7 +100,6 @@ class ironic::pxe (
     server      => '/usr/sbin/in.tftpd',
     socket_type => 'dgram',
     cps         => '100 2',
-    flags       => 'IPv4',
     per_source  => '11',
     wait        => 'yes',
     subscribe   => Anchor['ironic::install::end'],

spec/classes/ironic_pxe_spec.rb

@@ -64,7 +64,6 @@ describe 'ironic::pxe' do
        'server'      => '/usr/sbin/in.tftpd',
        'socket_type' => 'dgram',
        'cps'         => '100 2',
-       'flags'       => 'IPv4',
        'per_source'  => '11',
        'wait'        => 'yes',
        'subscribe'   => 'Anchor[ironic::install::end]',