Subnet delete for IPv6 SLAAC should not require prior port disassoc

With the current Neutron implementation, a subnet cannot be deleted
until all associated IP addresses have been remove from ports (via
port update) or the associated ports/VMs have been deleted.

In the case of SLAAC-enabled subnets, however, it's not feasible to
require removal of SLAAC-generated addresses individually from each
associated port before deleting a subnet because of the multicast
nature of RA messages. For SLAAC-enabled subnets, the processing of
subnet delete requests needs to be changed so that these subnets will
be allowed to be deleted, and all ports get disassociated from their
corresponding SLAAC IP address, when there are ports existing on
the SLAAC subnet.

Change-Id: I281f5a1553248e09174dc49d0a42aef4b5c44bee
Closes-Bug: 1393435
(cherry picked from commit ce5e95723925b0d7750bb3daa5be54765985cc30)
Merge "Subnet delete for IPv6 SLAAC should not require prior port disassoc" into stable/juno