Refactor spec.config to use GitLab Runner configuration template

Refactor the spec.config option in GitLab runner to use the GitLab Runner configuration template.

This approach gives the user a way to customize the GitLab Runner configuration giving them the ability customize a myriad of options.

To use the spec.config field to setup build volumes. Use the example below as a guide.

$ cat > custom.config.toml << EOF
[[runners]]
  [runners.kubernetes]
    [runners.kubernetes.volumes]
      [[runners.kubernetes.volumes.empty_dir]]
        name = "empty_dir"
        mount_path = "/var/configmap_1"
        medium = "Memory"
EOF
$ oc create configmap custom-config-toml --from-file config.toml=custom.config.toml
configmap/custom-config-toml created
$ oc get cm custom-config-toml -o yaml | yq r - data
config.toml: |
  [runners]]
    [runners.kubernetes]
      [runners.kubernetes.volumes]
        [[runners.kubernetes.volumes.empty_dir]]
          name = "empty_dir"
          mount_path = "/var/configmap_1"
          medium = "Memory"
$ oc get runner example -o yaml
apiVersion: apps.gitlab.com/v1beta2
kind: Runner
metadata:
  creationTimestamp: "2021-02-25T19:37:03Z"
  generation: 2
  name: example
  namespace: gitlab-runner-system
  resourceVersion: "1338690"
  selfLink: /apis/apps.gitlab.com/v1beta2/namespaces/gitlab-runner-system/runners/example
  uid: 70eb13a5-aae8-4f2a-8198-cbdfbe3397ea
spec:
  config: custom-config-toml
  gitlabUrl: https://gitlab.com
  imagePullPolicy: Always
  tags: openshift, test
  token: gitlab-dev-runner-secret
status:
  phase: Running
  registration: succeeded
$ oc exec -it example-runner-64fdb877fc-mz4sz -- cat /home/gitlab-runner/.gitlab-runner/config.toml | tail -n 10
    pod_annotations_overwrite_allowed = ""
    [runners.kubernetes.affinity]
    [runners.kubernetes.pod_security_context]
    [runners.kubernetes.volumes]

      [[runners.kubernetes.volumes.empty_dir]]
        name = "empty_dir"
        mount_path = "/var/configmap_1"
        medium = "Memory"
    [runners.kubernetes.dns_config]
$ 

Closes #23 (closed)