data/aws_iam_user: Add a data source for IAM user

[slaunay]

New feature

This data source can be used to fetch information about a specific IAM user.
By using this data source, you can reference IAM user properties without having to hard code ARNs or unique IDs as input (e.g. referencing a user aws:userId when creating an IAM policy).

Code change

• adding aws_iam_user data source
• adding acceptance test cases (existing user and non existing user)
• adding documentation for the new data source

Testing done

TF_ACC=1 go test -run=TestAccAWSDataSourceIAMUser_basic -v
=== RUN   TestAccAWSDataSourceIAMUser_basic
--- PASS: TestAccAWSDataSourceIAMUser_basic (11.09s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	11.118s

Misc

The code of this contribution was based a lot on #1140, thanks @stack72.

[radeksimko]

Hi @slaunay
we rarely test for negative cases (that something doesn't exist), because such tests tend to be very fragile. We should make no assumptions about the AWS account and IAM users (and their names) if possible.

For that reason I took the liberty and removed that test before merging as the PR looked otherwise simple and I thought it's worth merging straight away.

I also added a missing link to the sidebar. I hope you don't mind.

[slaunay]

👍 @radeksimko Thanks for making those changes and merging them.

The failure test helped me find out that the resp variable is never nil (after cloning the d/iam_group feature) when looking at the code coverage:
https://github.com/terraform-providers/terraform-provider-aws/blob/5815c0d72834326f2315c6e9ceaab460f93d1b77/aws/data_source_aws_iam_group.go#L46-L52

But I agree that the regexp makes the test brittle.

[ghost]

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