Adding subnet behavior methods for IPv6

- Adding netaddr to pip-requires. netaddr to be used from now on
instead of IPy (current IPy use should be replaced later on with
netaddr)
- Adding random cidr generation to subnet behavior method:
create_ipv6_cidr
- Adding get_allocation_pools subnet behavior method
- Adding format_allocation_pools subnet behavior method

Change-Id: I42a1685c12fc505885669dc2625ae0d2f3e5b43e
Merge "Adding subnet behavior methods for IPv6"