feat(opennebula): support ETHx_METHOD and ETHx_IP6_METHOD

[mcanevet]

Proposed Commit Message

feat(opennebula): support ETHx_METHOD and ETHx_IP6_METHOD

cloud-init's OpenNebula datasource always generated static Netplan
configuration, ignoring ETHx_METHOD and ETHx_IP6_METHOD entirely.

Add get_method() and get_ip6_method() helpers and rework the IPv4/IPv6
blocks in gen_conf() to branch on the configured method:

IPv4 (ETHx_METHOD):
- static (default): existing behaviour, no regression
- dhcp: emit dhcp4: true, no addresses
- skip: omit interface from Netplan output entirely

IPv6 (ETHx_IP6_METHOD):
- static (default when ETHx_IP6 or ETHx_IP6_GATEWAY is present):
  existing behaviour
- dhcp/dhcp6: emit dhcp6: true
- auto/slaac: emit accept-ra: true
- disable/skip: suppress all IPv6 config

Additional Context

Part of a series bringing cloud-init's OpenNebula datasource to feature
parity with the context-linux package from one-apps.

The get_ip6_method() helper infers the default IPv6 method from context:
if ETHx_IP6 or ETHx_IP6_GATEWAY is present, it defaults to static;
otherwise disable. This preserves backwards-compatible behaviour for all
existing tests.

Test Steps

In an OpenNebula VM context, set:

ETH0_METHOD="dhcp"

After boot, verify the interface obtains its IP via DHCP (no static
address in ip addr show).

For IPv6 SLAAC:

ETH0_IP6_METHOD="auto"

After boot, verify ip addr show shows a SLAAC-derived IPv6 address.

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)