provider/aws: Opsworks Agent has a default, needs to be computed

[catsby]

agent_version was introduced to Opsworks Stacks in #6493, however as an optional attribute that will have a default if not specified. This PR makes the attribute computed, if the user omits it. Fixes failing Acc tests:

--- FAIL: TestAccAWSOpsworksStackVpc (15.79s)
    testing.go:172: Step 0 error: After applying this step, the plan was not empty:

        DIFF:

        UPDATE: aws_opsworks_stack.tf-acc
          agent_version: "3437-20160504095744" => ""

        STATE:

[apparentlymart]

👍 LGTM!

[ghost]

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