Alicloud: allow use RAM role for OSS client

[bittopaz]

This is part of #4127.

This is the first step to gradually move from environment variables ACCESS KEY and SECRET to instance RAM role (known as IAM role in EC2).

This is also related with kopeio/etcd-manager#269. After this PR gets merged, I will try make another separate PR to update the kops dependency to latest in etcd-manager.

Then kopeio/etcd-manager#269 can be merged.

[k8s-ci-robot]

Hi @bittopaz. Thanks for your PR.

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[zetaab]

/ok-to-test

[zetaab]

/approve
/lgtm

[k8s-ci-robot]

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