installer: allow to specify storageClass in gitpod.yaml

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[jenting]

@sagor999
Now the installer contains the storageClass and snapshotClass in config.yaml already.
Then, the user could change the value before rendering the gitpod.yaml manifest and apply it to the Kubernetes cluster.

What else should we implement? If not, I'd like to close this issue.

[sagor999]

yup. this task was already done. close it please.