Migrate to use Invenio Helm Chart

A new Invenio Helm Chart has been created, see https://github.com/inveniosoftware/helm-invenio.

The template-openshift repository was used as a base for this work, however, the usage of ImageStreams has been cut out to simplify and finish within the time allocated for the Invenio sprint. The usage of ImageStreams is key to minimize the time to deploy hotfixes and security patches (builds on GitLab, automatically pushing to CERN OpenShift tags project etc..) so we should migrate the ImageStream part to Helm and update docs accordingly:

• Migrate ImageStreams to Helm templates and expose them as a configuration in values.yaml (make them optional)
• Update docs:
  • Steps 2, 3 and 4 are now replaced by Helm install.
  • Delete the "Fork this template" as this is not needed with Helm, we can parametrise everything within values.yaml.
  • Point to helm-invenio docs for all basic operations and leave here just what is CERN specific (all the GitLab specific scripts)