Verification done on MAAS 3.0.2.

In a HA deployment with 1 region+rack and 1 rack,
the dhcpd.conf file's section 'failover peer' has
IPv4+IPv6 address in 3.0.1 and only IPv4 in 3.0.2

The synthetic reproducer changes the order of IPs
in the maasdb so the IPv6 address has a _lower id_
than then IPv4 address (and thus goes first in the
for-loop in get_ip_address_for_rack_controller()).

(Steps are described in more detail in comment #3.)

Before DB changes:

	# maas admin rack-controllers read | jq '.[] | [.system_id, .hostname, (.interface_set[] | .links[] | [.id, .ip_address, .subnet.vlan.id])]'
	[
	  "hy37ba",
	  "maas-srv-30snap",
	  [
	    1,
	    "10.30.200.2",
	    5001
	  ],
	  [
	    3,
	    "fc00::2",
	    5001
	  ]
	]
	[
	  "8meq3e",
	  "maas-srv-peer-30snap",
	  [
	    2,
	    "10.30.200.3",
	    5001
	  ],
	  [
	    4,
	    "fc00::3",
	    5001
	  ]
	]

DB changes:

	# maas-test-db.psql
	postgres=# \c maasdb
	maasdb=# select id, ip from maasserver_staticipaddress order by id;
	 id |     ip
	----+-------------
	  1 | 10.30.200.2
	  2 | 10.30.200.3
	  3 | fc00::2
	  4 | fc00::3
	(4 rows)

	maasdb=# update maasserver_staticipaddress set ip='1.2.3.4' where id=4;
	UPDATE 1
	maasdb=# update maasserver_staticipaddress set ip='fc00::3' where id=2;
	UPDATE 1
	maasdb=# update maasserver_staticipaddress set ip='10.30.200.3' where id=4;
	UPDATE 1

	maasdb=# select id, ip from maasserver_staticipaddress order by id;
	 id |     ip
	----+-------------
	  1 | 10.30.200.2
	  2 | fc00::3
	  3 | fc00::2
	  4 | 10.30.200.3
	(4 rows)

	maasdb=# \q

After DB changes:

	# maas admin rack-controllers read | jq '.[] | [.system_id, .hostname, (.interface_set[] | .links[] | [.id, .ip_address, .subnet.vlan.id])]'
	[
	  "hy37ba",
	  "maas-srv-30snap",
	  [
	    1,
	    "10.30.200.2",
	    5001
	  ],
	  [
	    3,
	    "fc00::2",
	    5001
	  ]
	]
	[
	  "8meq3e",
	  "maas-srv-peer-30snap",
	  [
	    2,
	    "fc00::3",
	    5001
	  ],
	  [
	    4,
	    "10.30.200.3",
	    5001
	  ]
	]

MAAS 3.0.1:

	# snap list maas
	Name  Version                  Rev    Tracking    Publisher   Notes
	maas  3.0.1-10052-g.82c730c57  28871  3.0/stable  canonical✓  -

	Enable DHCP with HA:
	
	root@maas-srv-30snap:~# sudo cat /var/snap/maas/common/maas/dhcpd.conf | sed -n '/^failover peer/,/^}/p'
	failover peer "failover-vlan-5001" {
	    primary;
	    address 10.30.200.2;
	    peer address fc00::3;
	    max-response-delay 60;
	    max-unacked-updates 10;
	    load balance max seconds 3;
	    mclt 3600;
	    split 255;
	}

	root@maas-srv-peer-30snap:~# sudo cat /var/snap/maas/common/maas/dhcpd.conf | sed -n '/^failover peer/,/^}/p'
	failover peer "failover-vlan-5001" {
	    secondary;
	    address fc00::3;
	    peer address 10.30.200.2;
	    max-response-delay 60;
	    max-unacked-updates 10;
	    load balance max seconds 3;
	}

	Disable/Re-enable DHCP with HA (switch the order of servers for testing purposes):

	root@maas-srv-30snap:~# sudo cat /var/snap/maas/common/maas/dhcpd.conf | sed -n '/^failover peer/,/^}/p'
	failover peer "failover-vlan-5001" {
	    secondary;
	    address 10.30.200.2;
	    peer address fc00::3;
	    max-response-delay 60;
	    max-unacked-updates 10;
	    load balance max seconds 3;
	}

	root@maas-srv-peer-30snap:~# sudo cat /var/snap/maas/common/maas/dhcpd.conf | sed -n '/^failover peer/,/^}/p'
	failover peer "failover-vlan-5001" {
	    primary;
	    address fc00::3;
	    peer address 10.30.200.2;
	    max-response-delay 60;
	    max-unacked-updates 10;
	    load balance max seconds 3;
	    mclt 3600;
	    split 255;
	}

MAAS 3.0.2:

	# snap refresh --channel=3.0/edge maas
	# snap list maas
	Name  Version                  Rev    Tracking  Publisher   Notes
	maas  3.0.2-10056-g.089ec66c2  33607  3.0/edge  canonical✓  -

	Enable DHCP with HA:
	
	root@maas-srv-30snap:~# sudo cat /var/snap/maas/common/maas/dhcpd.conf | sed -n '/^failover peer/,/^}/p'
	failover peer "failover-vlan-5001" {
	    primary;
	    address 10.30.200.2;
	    peer address 10.30.200.3;
	    max-response-delay 60;
	    max-unacked-updates 10;
	    load balance max seconds 3;
	    mclt 3600;
	    split 255;
	}

	root@maas-srv-peer-30snap:~# sudo cat /var/snap/maas/common/maas/dhcpd.conf | sed -n '/^failover peer/,/^}/p'
	failover peer "failover-vlan-5001" {
	    secondary;
	    address 10.30.200.3;
	    peer address 10.30.200.2;
	    max-response-delay 60;
	    max-unacked-updates 10;
	    load balance max seconds 3;
	}

	Disable/Re-enable DHCP with HA (switch the order of servers for testing purposes):

	root@maas-srv-30snap:~# sudo cat /var/snap/maas/common/maas/dhcpd.conf | sed -n '/^failover peer/,/^}/p'
	failover peer "failover-vlan-5001" {
	    secondary;
	    address 10.30.200.2;
	    peer address 10.30.200.3;
	    max-response-delay 60;
	    max-unacked-updates 10;
	    load balance max seconds 3;
	}

	root@maas-srv-peer-30snap:~# sudo cat /var/snap/maas/common/maas/dhcpd.conf | sed -n '/^failover peer/,/^}/p'
	failover peer "failover-vlan-5001" {
	    primary;
	    address 10.30.200.3;
	    peer address 10.30.200.2;
	    max-response-delay 60;
	    max-unacked-updates 10;
	    load balance max seconds 3;
	    mclt 3600;
	    split 255;
	}

	Disable/Re-enable DHCP with HA (switch the order of servers for testing purposes):

	root@maas-srv-30snap:~# sudo cat /var/snap/maas/common/maas/dhcpd.conf | sed -n '/^failover peer/,/^}/p'
	failover peer "failover-vlan-5001" {
	    primary;
	    address 10.30.200.2;
	    peer address 10.30.200.3;
	    max-response-delay 60;
	    max-unacked-updates 10;
	    load balance max seconds 3;
	    mclt 3600;
	    split 255;
	}

	root@maas-srv-peer-30snap:~# sudo cat /var/snap/maas/common/maas/dhcpd.conf | sed -n '/^failover peer/,/^}/p'
	failover peer "failover-vlan-5001" {
	    secondary;
	    address 10.30.200.3;
	    peer address 10.30.200.2;
	    max-response-delay 60;
	    max-unacked-updates 10;
	    load balance max seconds 3;
	}