ec2: Do not enable DHCPv6 on EC2
    
    Enabling DHCPv6 by default on EC2 leads to instance with broken IPv6
    connectivity. AWS provides the IP address via DHCPv6 and SLAAC, but
    router advertisements are not accepted if the instance has DHCPv6
    configured. The accept_ra value is set to 0.
    
    Removing the `dhcp` option from `ipv6.method` and simply using `auto`
    (the default) allows the instance to pick up its IPv6 address and accept
    router advertisements from the AWS network.
    
    RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=2092459
    Pagure: https://pagure.io/cloud-sig/issue/382
    Upstream: https://bugs.launchpad.net/cloud-init/+bug/1976526?comments=all
    
    Signed-off-by: Major Hayden <major@redhat.com>