Mention about possible state store vendors in error message

[nak3]

When missed --state flag or specified invalid format, error message
asks us to use s3 bucket.

e.g:

$ kops create cluster --name foo --zones $ZONE  --project=${PROJECT} --state=foo

State Store: Invalid value: "foo": Unable to read state store s3 bucket.
Please use a valid s3 bucket uri when setting --state or KOPS_STATE_STORE env var.
A valid value follows the format s3://<bucket>.
Trailing slash will be trimmed.

The state store supports some vendors now, hence, this patch updates
the message.

[k8s-ci-robot]

Hi @nak3. Thanks for your PR.

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[nak3]

cc @chrisz100 @gambol99 as per #5931

[justinsb]

Thanks @nak3. I think we might have to regenerate the docs (make gen-cli-docs I believe), but the tests will check that!

/approve
/lgtm
/ok-to-test

[k8s-ci-robot]

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