sysconfig needs fix for ipv6 gateway routes

[Ryan Harper]

Static IPV6 subnet with a ipv6 route fails to set the default IPV6 gateway.

With cloud-init master:

% cat ipv6-static.yaml
network:
    version: 1
    config:
        - type: physical
          name: interface0
          mac_address: "52:54:00:12:34:00"
          subnets:
              - type: static
                address: 2001:4800:78ff:1b:be76:4eff:fe06:96b3
                netmask: 'ffff:ffff:ffff:ffff::'
                routes:
                  - gateway: 2001:4800:78ff:1b::1
                    netmask: '::'
                    network: '::'

% ./tools/net-convert.py --network-data ipv6-static.yaml \
  --kind yaml --output-kind sysconfig -d target-sysconfig

% grep -nr IPV6_DEFAULTGW target-sysconfig/; echo $?
1

Currently only the subnet is checked for 'ipv6' setting, however, the routes array may include a mix of v4 or v6 configurations, in particular, the gateway
in a route may be ipv6, and if so, should export the value via IPV6_DEFAULTGW in the ifcfg-XXXX file.

Additionally, if the route is v6, it should rendering a routes6-XXXX file; this is present but missing the 'dev <interface>' scoping.

% cat target-sysconfig/etc/sysconfig/network-scripts/route6-interface0
# Created by cloud-init on instance boot automatically, do not edit.
#
::/0 via 2001:4800:78ff:1b::1

[Scott Moser (smoser)]

This bug is believed to be fixed in cloud-init in 17.1. If this is still a problem for you, please make a comment and set the state back to New

Thank you.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/2913