Validate CR name

[jsafrane]

Only name: cluster is allowed. Using enum, because it has better error message than pattern:

With enum:

The Driver "clusterX" is invalid: metadata.name: Unsupported value: "clusterX": supported values: "cluster"

With pattern: "^cluster$":

The Driver "clusterX" is invalid: metadata.name: Invalid value: "": metadata.name in body should match '^cluster$'

Fixes: #21

[gnufied]

/lgtm

[openshift-ci-robot]

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