ntp: Work around Labs network error

On Labs the default "interface" configuration leads to ntpd
complaining that it can't create a IPv6 socket for eth0.  To work
around that, we let ntpd listen only on IPv4.

Bug: 60166
Change-Id: Ia88671965bb447bf306e97685ae35472cae721a9
Merge "ntp: Work around Labs network error" into production