Handle IPv6 DHCP allocations

The code for generating the dnsmasq config had a bad python string
format expression, causing an unhandled exception when any IPv6 subnet
had DHCP enabled.

Fixes DHC-2126

Change-Id: Iede9fe7179911bb6df520696a9b978a288d7a5d4
Merge pull request #49 from dhellmann/bug/dhc-2126

Handle IPv6 DHCP allocations