Update External Obect Storage Documentation to include `gitlab-runner.runners.cache` confuguration

The Current documentation for External Object Storage does not state that gitlab-runner.runners.cache should be set when global.mino.enabled: false. The example file also misses this.

The gitlab-runner pod fails with an error like below despite setting minio to false:

Unable to mount volumes for pod "gitlab-gitlab-runner-5958c67976-k8thq_default(5c3c15dd-c335-11e8-968e-42010a8001f4)": timeout expired waiting for volumes to attach/mount forpod "default"/"gitlab-gitlab-runner-5958c67976-k8thq". list of unattached/unmounted volumes=[init-runner-secrets]
  Warning  FailedMount            2m (x9 over 4m)  kubelet, gke-abubakar-runner-demo-default-pool-7039f19a-njdh  MountVolume.SetUp failed for volume "init-runner-secrets" : secrets "gitlab-minio-secret" not found

Also, the commandline-options for setting the right options are only available for S3 and none for GCS, despite such configuration available for GCS in Runner Advanced Configuration.

/cc @WarheadsSE