pod startup: pre-define generic ephemeral volume type

[pohly]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This can be used to create pods with a varying number of generic ephemeral
inline volumes.

Example for usage with the "distributed" deployment of csi-driver-host-path:

  $ cat test.yaml
  PODS_PER_NODE: 5
  VOLUMES_PER_POD: 3
  VOL_SIZE: 1Gi
  STORAGE_CLASS: csi-hostpath-fast

  $ go run cmd/clusterloader.go -v=3 --kubeconfig=$KUBECONFIG --provider=local --nodes=1 \
       --testconfig=testing/experimental/storage/pod-startup/config.yaml \
       --testoverrides=testing/experimental/storage/pod-startup/volume-types/genericephemeralinline/override.yaml \
       --testoverrides=test.yaml

Which issue(s) this PR fixes:

Related-to: kubernetes/enhancements#1698

Does this PR introduce a user-facing change?:

the experimental pod startup test now also has a pre-defined volume type for generic ephemeral volumes

[wojtek-t]

/lgtm
/approve

Thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pohly, wojtek-t

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