Try to move Jenkins folder to "conan-io/conan_ci_jenkins"

In this repo we can find tests, but also a `Jenkinsfile` and helpers to run the tests (folder `jenkins`). It would make sense to move these files to repo [conan-io/conan_ci_jenkins](https://github.com/conan-io/conan_ci_jenkins) and maintain here only the actual tests. Merging these sources into that repo should avoid some code duplication.