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 canonical/cloud-init#1045