No allocation needed for specific IPv6 SLAAC addr assignment

No allocation needed for specific IPv6 SLAAC addr assignment

(Patch set #7 for the multiple-ipv6-prefixes blueprint)

On internal router ports, Neutron allows for an address to
be assigned for an IPv6 SLAAC subnet that is not necessarily
EUI-64. This makes it easier for subnet create, since a
convenient address, e.g. one ending in ::1, can be used as
the subnet gateway IP address.

Currently, when an internal router port is created with a specific
(non-EUI-64) address for a SLAAC subnet, the call flow includes
a call to _allocate_specific_ip. This call is not necessary,
since we're not allocating an address from a pool (and
recalibrating availability ranges, etc.).

This patch set prevents the call to _allocate_specific_ip for
this scenario.

Co-Authored-By: Baodong (Robert) Li <baoli@cisco.com>
Change-Id: I2533ee82980bb602faa663b875787ca50b268b34
Partially-implements: blueprint multiple-ipv6-prefixes