haproxy is listening on ipv4 when ipv6 enabled

[Edward Hope-Morley]

global
    log 127.0.0.1 local0
    log 127.0.0.1 local1 notice
    maxconn 20000
    user haproxy
    group haproxy
    spread-checks 0

defaults
    log global
    mode tcp
    option tcplog
    option dontlognull
    retries 3
    timeout queue 1000
    timeout connect 1000
    timeout client 30000
    timeout server 30000

listen stats :8888
    mode http
    stats enable
    stats hide-version
    stats realm Haproxy\ Statistics
    stats uri /
    stats auth admin:password

listen dash_insecure 0.0.0.0:80
    balance source
    option tcplog
    server openstack-dashboard-1 2001:db8:1:0:f816:3eff:fe81:70b0:70 check
    server openstack-dashboard-0 2001:db8:1:0:f816:3eff:fe6d:4f0f:70 check

listen dash_secure 0.0.0.0:443
    balance source
    option tcplog
    server openstack-dashboard-1 2001:db8:1:0:f816:3eff:fe81:70b0:433 check
    server openstack-dashboard-0 2001:db8:1:0:f816:3eff:fe6d:4f0f:433 check