Distro-specific network rendering

@bin456789 originally reported here: https://github.com/canonical/cloud-init/pull/4591#issuecomment-1816551911

###

> I found another bug in sysconfig.py
> This line should be removed.
> https://github.com/canonical/cloud-init/blob/29ed5f5b646eebfd2dafa7e036e813516a8b99bf/cloudinit/net/sysconfig.py#L440C14-L440C14

>> What's the bug?

> ```
> if subnet_type == "dhcp6" or subnet_type == "ipv6_dhcpv6-stateful":
> if flavor == "suse":
> ...
> # only if rhel AND dhcpv6 stateful
> elif flavor == "rhel" and (
> # this line should be subnet_type in ("ipv6_dhcpv6-stateful" or "dhcp6" )
> # but that's redundant so it's good to remove it
> ↓↓↓
> subnet_type == "ipv6_dhcpv6-stateful"
> ):
> iface_cfg["DHCPV6C"] = True
> iface_cfg["IPV6INIT"] = True
> iface_cfg["IPV6_AUTOCONF"] = False
> iface_cfg["IPV6_FAILURE_FATAL"] = True
> # maybe remove follow lines if these's no other flavor else than suse/rhel
> ↓↓↓
> else:
> iface_cfg["IPV6INIT"] = True
> # Configure network settings using DHCPv6
> iface_cfg["DHCPV6C"] = True
> ```

>> can you please file a separate bug and include an example output configuration that is incorrect as a result of these lines with an explanation of why it is wrong? Your initial explanation and reproducer on this PR was an example of a good bug report.

config 1:
dhcp + dhcp6
```
root@ubuntu:/# cat test.cfg
#cloud-config
datasource_list: [NoCloud]
network:
version: 1
config:
- type: physical
name: eth0
mac_address: 56:00:04:a2:c5:f0
subnets:
- type: dhcp
- type: dhcp6
```
```
root@ubuntu:/# cloud-init devel net-convert -p test.cfg -k yaml -d out -O sysconfig -D centos
Read input format 'yaml' from 'test.cfg'.
Wrote output format 'sysconfig' to 'out/'
```
```
root@ubuntu:/# cat out/etc/sysconfig/network-scripts/ifcfg-eth0
# Created by cloud-init automatically, do not edit.
#
BOOTPROTO=dhcp
DEVICE=eth0
DHCPV6C=yes
HWADDR=56:00:04:a2:c5:f0
IPV6INIT=yes
NM_CONTROLLED=no
ONBOOT=yes
TYPE=Ethernet
USERCTL=no
```

config 2:
dhcp + ipv6_dhcpv6-stateful
```
root@ubuntu:/# cat test.cfg
#cloud-config
datasource_list: [NoCloud]
network:
version: 1
config:
- type: physical
name: eth0
mac_address: 56:00:04:a2:c5:f0
subnets:
- type: dhcp
- type: ipv6_dhcpv6-stateful
```
```
root@ubuntu:/# cloud-init devel net-convert -p test.cfg -k yaml -d out -O sysconfig -D centos
Read input format 'yaml' from 'test.cfg'.
Wrote output format 'sysconfig' to 'out/'
```
```
root@ubuntu:/# cat out/etc/sysconfig/network-scripts/ifcfg-eth0
# Created by cloud-init automatically, do not edit.
#
BOOTPROTO=dhcp
DEVICE=eth0
DHCPV6C=yes
HWADDR=56:00:04:a2:c5:f0
IPV6INIT=yes
IPV6_AUTOCONF=no
IPV6_FAILURE_FATAL=yes
NM_CONTROLLED=no
ONBOOT=yes
TYPE=Ethernet
USERCTL=no
```

They should produce the same result, but they're not at the moment.