vyatta-cfg-system: add 'set system allow-dhcp-nameservers' option

Instead of simply allowing / denying DHCP related updates to resolv.conf
based on the current values of 'set system name-server', as initially
proposed for Bug #182 (http://bugzilla.vyos.net/show_bug.cgi?id=182),
this patch replaces that behaviour with a global option to allow /
deny these updates.

Add 'set system allow-dhcp-nameservers' as a boolean value that has the
default value of true, so allowing DHCP nameserver updates by default.

Bug #308 http://bugzilla.vyos.net/show_bug.cgi?id=308