Add option to run UBI8-minimal based stack images

[axw]

Now that elastic/apm-server#4066 is implemented, we should update apm-integration-testing with an option to run the UBI8-minimal based images.

[v1v]

UBI docker images generation was enabled in elastic/beats#20576

Docker images for the apm-server were generated as expected in the apm-ci:

• See gcs objects

And also available in the internal docker registry:

$ docker inspect --format "{{ json .Config.Labels }}" docker.elastic.co/apm/apm-server-ubi8:8.0.0-SNAPSHOT  | jq .
{
  "architecture": "x86_64",
  "build-date": "2020-09-01T19:43:00.932453",
  "com.redhat.build-host": "cpt-1003.osbs.prod.upshift.rdu2.redhat.com",
  "com.redhat.component": "ubi8-minimal-container",
  "com.redhat.license_terms": "https://www.redhat.com/en/about/red-hat-end-user-license-agreements#UBI",
  "description": "Elastic APM Server",
  "distribution-scope": "public",
  "io.k8s.description": "Elastic APM Server",
  "io.k8s.display-name": "Apm-Server image",
  "io.openshift.expose-services": "",
  "io.openshift.tags": "minimal rhel8",
  "license": "Elastic License",
  "maintainer": "infra@elastic.co",
  "name": "apm-server",
  "org.label-schema.build-date": "2020-09-20T06:56:06Z",
  "org.label-schema.license": "Elastic License",
  "org.label-schema.name": "apm-server",
  "org.label-schema.schema-version": "1.0",
  "org.label-schema.url": "https://www.elastic.co/products/apm",
  "org.label-schema.vcs-ref": "adfce945421beeb535f60bfbe64ba1690bc9dc65",
  "org.label-schema.vcs-url": "github.com/elastic/apm-server",
  "org.label-schema.vendor": "Elastic",
  "org.label-schema.version": "8.0.0",
  "org.opencontainers.image.created": "2020-09-20T06:56:06Z",
  "org.opencontainers.image.licenses": "Elastic License",
  "org.opencontainers.image.title": "Apm-Server",
  "org.opencontainers.image.vendor": "Elastic",
  "release": "1",
  "summary": "apm-server",
  "url": "https://www.elastic.co/products/apm",
  "vcs-ref": "34bc4001b02c4f75e89663e1f8407a08af05f8ab",
  "vcs-type": "git",
  "vendor": "Elastic",
  "version": "8.0.0"
}