Make NTP server configurable

[peter-mount]

Currently NTP is hardcoded to pool.ntp.org which means that whenever the clock needs to be resynced it involves accessing a service outside of the local network.

For instances where either the internet is not always accessible, or firewalled, some environments have their own ntp servers. Others might use a different ntp pool.

So a means of enabling an alternate ntp server other than the hardcoded one would be useful.

[GeoffMurray]

I changed the definition line in npt.py to:

def fetch(synch_with_rtc=True, timeout=10, ntp_host="pool.ntp.org"):
#   ntp_host = "pool.ntp.org"

And then called it like this:

timestamp = ntp.fetch(True, 10, "ntp-b.nist.gov")

Seems to work.

[jimkoke]

 DEFAULT_NTP_HOST = "pool.ntp.org"
 def fetchFromPool(synch_with_rtc=True, timeout=10, tzinfo=0,
            pool = (DEFAULT_NTP_HOST,)):
   for ntp_host in pool:
      ts = fetch(synch_with_rtc=synch_with_rtc, timeout=timeout,
              tzinfo=tzinfo, ntp_host=ntp_host)
      if ts is not None:
         return ts
   return None
 ...
 ts = fetchFromPool(True, 10, ( "ntp-b.hist.gov", DEFAULT_NTP_HOST ) )