Comment 0 for bug 1806014

When instructing cloud-init to use `type: dhcp6`, the end-result is that `/etc/sysconfig/network` contains `IPV6_AUTOCONF=no`. This causes router advertisements to be ignored and no default route set up.

https://github.com/cloud-init/cloud-init/blob/master/cloudinit/net/network_state.py#L333
This configures `network.use_ipv6 = True`, which is fine.

https://github.com/cloud-init/cloud-init/blob/master/cloudinit/net/sysconfig.py#L649
This configures `IPV6_AUTOCONF=no` to be written whenever `network.use_ipv6` is `True`.
However, in the use-case where dhcp6 is used, `IPV6_AUTOCONF` should be set to `yes`.

Proposed fix:
Distinguish between IPv6 being configured statically or using DHCP when writing the sysconfig file.

Cloud-provider: AWS EC2.
Guest OS: CentOS Linux release 7.5.1804 (Core)
EC2 user-data provided with cloud-init config:
```
#cloud-config
package_upgrade: true
write_files:
- path: /etc/cloud/cloud.cfg.d/99-custom-networking.cfg
  owner: root:root
  permissions: 0600
  content: |
    network:
      version: 1
      config:
      - type: physical
        name: eth0
        subnets:
          - type: dhcp
          - type: dhcp6
power_state:
  mode: reboot
  delay: now
  message: Rebooting post-config
  timeout: 30
  condition: True
```