support nebula in feature environment

Introduction

This merge request incorporates the nebula overlay network into the pipeline.

The description of the architecture can be seen here: https://gitlab.com/nunet/test-suite/-/tree/96-automate-configuration-of-nebula-overlay-network-in-lxd-vm-s/doc/architecture/feature_environment?ref_type=heads

Changes

• fix feature env pipeline not being able to pull the debian file from the Build job when testing
• only running one ubuntu version in the feature environment when testing from the test-suite.
• only running feature env jobs with tag nebula which restricts job runners that have nebula preconfigured
  • these runners were configured manually and the tags were added in the CICD group runners in nunet
• add nebula users to the config.yml.dist spec file in infrastructure/dms-on-lxd
• producing a new file vms-ipv4-list.txt that serves as inventory for nebula ipv4
• implemented basic functional behave tests using SSH as a proof of concept
• moved the actual config.yml file for terraform lxd from the gitlab variables to the vault
• added an ADR that specifies the adoption of SSH for remote code execution over lxc exec
• added svg file renderer using nunet/gitlab-ci/templates> repository
• added documentation with diagrams for the final architecture of the feature environment

Testing

These changes were showcased in the technical discussion from last thursday (2024-06-20). Reviewers can refer to that presentation. If needed, though, we should be able to join synchronously in a call to discuss these changes, as they are fairly involved in how they are supposed to run, since there are many pre-requisites and many moving parts that must come together in order to provide the functionality implemented in this MR.

Closes #96