vyatta-cfg-system: allow dhcp and dhcpv6 addresses to be deleted

Allow interfaces with either dhcp or dhcpv6 addresses to be deleted.
Expand on the previous patch for Bug #305 to now take dhcpv6 addresses
into account, as well as an update to the original dhcp logic.

Bug #341 http://bugzilla.vyos.net/show_bug.cgi?id=341
Merge pull request #26 from cyclops8456/helium

vyatta-cfg-system: allow dhcp and dhcpv6 addresses to be deleted