Add .golangci.yml linter configuration

As part of #385 (closed) we have introduced the use of a custom .golangci.yml file with some custom rules for linting.

This replaces the need of downloading and using golint, gofmt, go vet and gocyclo manually. We take advantage of the custom golangci-lint docker image as stated in the Automatic linting section of the Go standards and style guidelines.

This iteration enables a subset of linters, with the remaining of them enabled on a separate MR as described in the issue above.

The main changes introduced by this linter include:

• gosec: potential hardcoded credentials
• goconst: DRY by declaring and using constants
• gosimple: reduce statements complexity and improve return statements