master: migrate the CI integration away from Travis to GH Actions

[yeoldegrove]

This migrates the CI integration away from Travis to GH Actions. It is a port of #719 (develop branch) and later changes to the CI.

• It adds parallel builds of the different CI jobs.
• jobs:
  • codespell - spell checking
  • shellcheck - script linting
  • yamllint
  • jsonlint
  • mlc (markdown link checker)
  • salt-lint
  • terraform format
  • terraform validation