Enable dev builds

• Move packer image from globals to reusable .packer section
• Only run validate for pre-merge CI
• Save packer image manifest for later reuse
• Use branch name in image tags
• Add script to import cloud images into OpenStack
• Add pipeline stage to upload base OS image
• Merge before_script into .packer template
• Add pre-commit lint to pipeline
• Build on top of base OS image
• Explicitly build the runner in the runner job
• Drop the environment keyword
  It keeps the CI yaml simple and doesn't really make much sense within the context of packer anyway.
• Update default build rules
  Jobs are now:
  • Enabled on dev pipelines, but are always manual.
  • Run automatically upon merge to the default branch.
• Only run merge request and master pipelines