Crash when "restrict_networks" is null in config.yaml

[jathanism]

When restrict_networks is null in config.yaml, the following happens:

$ tail nsot.log
    main(sys.argv)
  File "/srv/nsot/envs/nsot/bin/nsot-server", line 37, in main
    settings.update_from_config(args.config)
  File "/srv/nsot/envs/nsot/lib/python2.7/site-packages/nsot/settings.py", line 33, in update_from_config
    value = override(value)
  File "/srv/nsot/envs/nsot/lib/python2.7/site-packages/nsot/settings.py", line 56, in override_restrict_networks
    new_values.append(ip_network(value))
  File "/srv/nsot/envs/nsot/lib/python2.7/site-packages/ipaddress.py", line 148, in ip_network
    address)
ValueError: u'None' does not appear to be an IPv4 or IPv6 network

We should allow a null value to equate to the defaults, perhaps?

[jathanism]

Oops, that's None as a string 'None'...