Create new IPv6 attributes for Subnets

Create new IPv6 attributes for Subnets

* Introduces two new optional attributes for Subnets:
  * ipv6_ra_mode
  * ipv6_address_mode

Both attributes accept the following values:

* dhcpv6-stateful
* dhcpv6-stateless
* slaac

In addition to these values, additional behaviors are specified for
when only one of the attributes is set. For example, a Neutron network
that uses a physical router as a gateway, that transmits ICMPv6 Router
Advertisement packets to configure hosts on the network will create
Neutron Subnets that have ipv6_ra_mode *not* set, and ipv6_address_mode
set to 'slaac' so that Neutron will calculate EUI64 addresses for
each port assigned to the subnet, and not spawn a Dnsmasq process.

These attributes maintain backwards compatability with the enable_dhcp
Subnet attribute, by requiring a subnet with these attributes to also
have enable_dhcp set to True.

DocImpact

Implements bp ipv6-two-attributes
Change-Id: I5b2313fff5dca1c16ff939fdc4397d7f95ba3ba5