Configures the ntp-server package to keep the local time synchronized.

The "server" class targets the central time servers of a
organisation or for single servers, directly synchronising to some online
servers.

The "client" class uses the defines "server"s to get the time, thereby
not burdening the public servers further.

Refer to http://www.eecis.udel.edu/~mills/ntp/html/notes.html for details
pertaining to a efficient and robust NTP Subnet implementation.

The upstream NTP servers are set by specifying them in an array called
$ntp_servers. It is recommended to use a different set of upstreams for each
"server". Are no upstreams specified, the Pool is used.

The stratum of the local clock is set via the $ntp_local_stratum variable, which
is 13 by default. One Server should have this set to a lower stratum to provide
a stable fallback in case of catastropic connection loss.