Lambda S3 object version defaults to '$LATEST' if unspecified

[radeksimko]

This is a rebased and polished version of #4770

Essentially I just resolved conflicts with some latest changes in the Lambda Function acceptance tests and replaced rand.New(rand.NewSource(time.Now().UnixNano())).Int() with acctest.RandInt().

The original credit obviously goes to @Bowbaq 😉

I will merge this myself once Travis turns green since it's just polished version of someone else's PR and the changes were quite trivial, but feel free to do code review afterwards.

I also ran acceptance tests:

TF_ACC=1 go test ./builtin/providers/aws -v -run=AWSLambdaFunction -timeout 120m
=== RUN   TestAccAWSLambdaFunction_basic
--- PASS: TestAccAWSLambdaFunction_basic (30.24s)
=== RUN   TestAccAWSLambdaFunction_VPC
--- PASS: TestAccAWSLambdaFunction_VPC (33.40s)
=== RUN   TestAccAWSLambdaFunction_s3
--- PASS: TestAccAWSLambdaFunction_s3 (32.21s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    95.874s

[ghost]

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