As part of #385
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 lintinb]
(https://docs.gitlab.com/ee/development/go_guide/#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