feat(opennebula): support ETHx_ALIASn_IP/MASK for anycast addresses

[mcanevet]

Proposed Commit Message

```
feat(opennebula): support ETHx_ALIASn_IP/MASK for anycast addresses

OpenNebula's context-linux package supports per-NIC alias addresses
(ETHx_ALIAS0_IP, ETHx_ALIAS1_IP, …) used for anycast IPs. Add
get_alias_addresses() to OpenNebulaNetwork and wire it into gen_conf()
so alias addresses appear in the Netplan v2 output alongside the
primary address. Missing MASK defaults to /32.
```

Additional Context

OpenNebula allows attaching multiple IP addresses to a single NIC via
ETH<x>_ALIAS<n>_IP / ETH<x>_ALIAS<n>_MASK context variables. These
are typically used for anycast or VIP addresses. Previously cloud-init
ignored them, leaving the addresses unconfigured on the guest.

Test Steps

1. In OpenNebula, add alias addresses to a NIC context:
   ```
   ETH0_ALIAS0_IP = "192.168.1.10"
   ETH0_ALIAS0_MASK = "255.255.255.0"
   ETH0_ALIAS1_IP = "192.168.1.11"
   ```
2. Boot a cloud-init enabled VM using that context.
3. Verify the alias addresses appear on the interface:
   ```
   ip addr show eth0

   expected: 192.168.1.10/24 and 192.168.1.11/32 present

   ```
4. Verify a NIC without aliases is unaffected.

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>)

[Copilot]

Pull request overview

This PR extends the OpenNebula datasource’s network configuration generation to include per-NIC alias (anycast/VIP) IPv4 addresses defined via ETH<x>_ALIAS<n>_IP / ETH<x>_ALIAS<n>_MASK, so they appear in the Netplan v2 addresses list alongside the primary address (defaulting missing masks to /32).

Changes:

• Add OpenNebulaNetwork.get_alias_addresses() to collect alias IPv4 addresses from context.
• Extend OpenNebulaNetwork.gen_conf() to append alias addresses into Netplan addresses.
• Add unit tests and update OpenNebula datasource documentation for the new context variables.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
cloudinit/sources/DataSourceOpenNebula.py	Implements alias address parsing and includes aliases in generated Netplan output.
tests/unittests/sources/test_opennebula.py	Adds unit tests covering alias parsing and integration into gen_conf().
doc/rtd/reference/datasources/opennebula.rst	Documents ETH<x>_ALIAS<n>_IP/MASK behavior and default mask handling.

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