rh_interface: More tolerant handling of IPv6 sysctl.

Make use of the new 'ignoremissing' parameter to sysctl, to slightly
better handle VLAN interfaces, which don't exist until they are brought
up by ifup(8).

This is not a full solution for the problem, it just works around one
particular error situation.

This was originally commit 14943d53d08f in nsc-puppet-utils.

manifests/init.pp

@@ -245,6 +245,7 @@ define rh_interface($bootproto='static',# 'static', 'dhcp' or 'unconfigured'
 	    sysctl::exec {
 		"net.ipv6.conf.${sysctl_ifname}.disable_ipv6":
 		    value => $ipv6_sysctl_value,
+		    ignoremissing => true,
 		    require => Exec[$ifconfig_exec_id];
 	    }
 	}