Update Jenkins docker build image: part 1

This is part one of the larger task (i.e. FOLIO-1940 to update all tools provided by the "jenkins-slave-all" docker build image).

For the past few months we have not been able to rebuild the image.

Its Dockerfile (at folio-tools/jenkins-slave-docker/Dockerfile) does pin some tools to specific versions, while others will automatically become the latest available. That is generally okay.

However one situation is preventing a rebuild. Chrome is only available as its most recent version, and when we try to rebuild jenkins-slave-all image, then the invocation of Karma for regression tests fails with that Chrome. Our tests do show that using the current Chromium instead is fine (FOLIO-2115).

Therefore we cannot update Chrome, other supporting tools such as aws-cli, and the other items from our folio-tools such as lint-raml and generate-api-docs.

This first part is intended to regain the ability to rebuild the image more frequently. The complete upgrade can then happen later.

The strategy is:

1) Leave the base OS image as-is (ubuntu:xenial). Leave other pinned tools as-is.

2) Update Yarn pinned version to be more recent.

3) Pin Ansible to the version that we currently use (2.7.x) – FOLIO-2156.

4) Use Chromium rather than Chrome – FOLIO-2115.