Gelencsér Szabolcs / circlestack

Feature network gui improvements

* closes #358 Automatically select a free ipv4 address based on vlan (host creation)
* closes #357 Automatically generate ipv6 address from ipv4 (host creation/edit)
* closes #354 MAC address autocomplete
* closes #360 Add sensible default and help_text to Vlan.ipv6_template

See merge request !273