Rename DLE to DBLab Engine and resolve tasks from TODO list (part 1)

• Rename DLE to DBLab Engine (issue: #13 (closed))
  • all dle_ variables renamed to dblab_engine_
  • except for platform variables, new names: platform_project_name, platform_org_key, platform_url
    • attention: snippets need to be updated on the platform - https://gitlab.com/postgres-ai/platform/-/issues/257
  • role "dle" renamed to "dblab-engine"
  • role "cli" renamed to "dblab-cli"
• Rename playbooks (issue: #1 (closed))
  • Role "provision" renamed to "cloud-resources"
  • Playbook "provision.yml" renamed to "cloud_resources.yml"
  • Added new playbook "software.yml"
    • Note: "deploy_dle.yml" playbook performs both playbooks "cloud_resources.yml" and "software.yml"
• Temporary SSH key name should be unique (issue: #8 (closed), commit 1df0bf75)
• Help Vim users (issue: #2 (closed), commit c13e17f1)