vyos.template: T5869: first_host_address() does not honor RFC4291 section 2.6.1
The subnet router anycast address is predefined. Its format is as follows:
| n bits | 128-n bits |
+------------------------------------------------+----------------+
| subnet prefix | 00000000000000 |
+------------------------------------------------+----------------+
The "subnet prefix" in an anycast address is the prefix that identifies a
specific link. This anycast address is syntactically the same as a unicast
address for an interface on the link with the interface identifier set to zero.
Packets sent to the Subnet-Router anycast address will be delivered to one
router on the subnet. All routers are required to support the Subnet-Router
anycast addresses for the subnets to which they have interfaces.
The Subnet-Router anycast address is intended to be used for applications where
a node needs to communicate with any one of the set of routers.
Our code as of now returns the subnet router anycast address as the
first_host_address().
(cherry picked from commit cc4ce81ece57faca8ce111b8f3748389ecb40202)