provider/aws: Add support for iam_openid_connect_provider

[radeksimko]

Closes #6137

In the spirit of reducing number of old, stale PRs and to speed things up a bit I took the liberty to make changes to the original PR and resubmitted.

@ryansroberts I hope you don't mind. Your contribution is still included (and still credited to you), with the appropriate modifications.

Feel free to ask about any of the changes.

Test plan

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSIAMOpenIDConnectProvider_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/06 12:14:51 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSIAMOpenIDConnectProvider_ -timeout 120m
=== RUN   TestAccAWSIAMOpenIDConnectProvider_basic
--- PASS: TestAccAWSIAMOpenIDConnectProvider_basic (68.43s)
=== RUN   TestAccAWSIAMOpenIDConnectProvider_importBasic
--- PASS: TestAccAWSIAMOpenIDConnectProvider_importBasic (38.50s)
=== RUN   TestAccAWSIAMOpenIDConnectProvider_disappears
--- PASS: TestAccAWSIAMOpenIDConnectProvider_disappears (54.81s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	161.773s

[stack72]

LGTM! Nice work on closing off this old PR :)

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.