Changes to enable volume mode conversion feature in prow jobs

[RaunakShah]

This PR does the following:

• Updates snapshot-controller manifest to enable --prevent-volume-mode-conversion if VOLUME_MODE_CONVERSION_TESTS is passed to the prow job.
• Refactor install_snapshot_controller() in prow.sh

Related to https://github.com/kubernetes-csi/csi-driver-host-path/pull/379 and https://github.com/kubernetes-csi/external-snapshotter/pull/790

Update prow.sh to enabled --prevent-volume-mode-conversion if VOLUME_MODE_CONVERSION_TESTS is set 

[k8s-ci-robot]

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[RaunakShah]

/hold

[RaunakShah]

Opened https://github.com/kubernetes-csi/external-provisioner/pull/832. Unfortunately lost the dummy tests I was running to test this PR. Will redo that and update the PR description accordingly. Closing this PR.