Since Amazon provides NTP servers within their network, this should
be far superior to using the default NTP pools; and since the service
is provided by Amazon there's very little risk in enabling it by
default. (If someone is able to compromise Amazon's NTP servers and
exploit them to attack EC2 instances, they would almost certainly be
able to compromise EC2 instances even without ntpd running...)
MFC after: 1 week
Relnotes: EC2 instances now keep their clocks synchronized using
the Amazon Time Sync Service (aka. NTP).