Do not force set ntp servers

[directionless]

There are cases where one does not want any ntp servers. Perhaps a node is using pools, or perhaps it's only using peers.

However, #170 changed the behavior here. As
implemented in https:/rhass/ntp/blob/master/recipes/default.rb#L61-L69

if node['ntp']['servers'].empty?
  node.default['ntp']['servers'] = [
    '0.pool.ntp.org',
    '1.pool.ntp.org',
    '2.pool.ntp.org',
    '3.pool.ntp.org',
  ]
  Chef::Log.debug 'No NTP servers specified, using default ntp.org server pools'
end

I'm not sure this was intentional or accidental.

[iennae]

Thanks for reporting. It looks like an unintended side effect of fixing this on mac os.

[directionless]

I might have misblamed #170, this may have always been true (which would represent a bug with my initial pools PR)

Regardless, I have fixed this in #178