Optional read secret from mount path

[aidun]

This PR implements the changes of #3

MINIO_SECRET_KEY is the default Source of the Secret. If MINIO_SECRET_KEY_MOUNT_PATH is present, minio-kv try to read the secret from the file provided in MINIO_SECRET_KEY_MOUNT_PATH. If there is any error, the fallback is MINIO_SECRET_KEY.

Starting Minio:

Building + Starting minio-kv:

Testing:

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[derek[bot]]

Thank you for your contribution. I've just checked and your commit doesn't appear to be signed-off. That's something we need before your Pull Request can be merged. Please see our contributing guide. Tip: if you only have one commit so far then run: git commit --amend --signoff and then git push --force.

[aidun]

I think there should be an input validation for the other environments to.

[aidun]

@alexellis is the implementation the way you want it?