Map Persistent Volume Claims as mounted volumes in the build container

[chickenandpork]

This PR allows the gitlab-runner to map Persistent Volume Claims to the runner's build container by leveraging runners.kubernetes.volumes.pvc blocks.

For example, I wanted a cache for a locally-running "Allure" test utility; I created a PVC, and configured it to mount as:

  ...
  resource "kubernetes_persistent_volume" "allure_results" {
    metadata {
      name = "allure-results"
    }
    ...
  }

  ...

  build_job_pvcs = {
    "/var/run/allure" = { name = "${kubernetes_persistent_volume.allure_results.metadata.0.name}" }
  }
  ...

This of course creates the config.toml section as follows:

  [[runners.kubernetes.volumes.pvc]]
    name = "allure-results-0"
    mount_path = "/var/run/allure"

Of course this is https://docs.gitlab.com/runner/executors/kubernetes.html#pvc-volumes in the docs and I haven't needed to flesh out read_only nor sub_path but extending it shouldn't be too difficult.