Update toolchain to use IP6_GATEWAY and not GATEWAY6

[rsmontero]

Description This issue is to change all opennebula components to rely only in IP6_GATEWAY context variable and not in GATEWAY6 This includes:

• onedb upgrade to change affected components (vnet)
• Sunstone
• OCA
• ...

Use case Consistent IPv6 context naming

Interface Changes Contextualization, Virtual Network data model.

Additional Context NaN

Progress Status

• [ ] Branch created
• [ ] Code committed to development branch
• [ ] Testing - QA
• [ ] Documentation
• [ ] Release notes - resolved issues, compatibility, known issues
• [ ] Code committed to upstream release/hotfix branches
• [ ] Documentation committed to upstream release/hotfix branches

[vholer]

PR for the https://github.com/canonical/cloud-init/pull/1045