* Adding import to resource_aws_iam_server_certificate.
* provider/aws: Update tests for import of aws_iam_server_certificate
Builds upon the work of @mrcopper in #12940
Resource:
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSIAMServerCertificate_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/25 00:08:48 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSIAMServerCertificate_ -timeout 120m
=== RUN TestAccAWSIAMServerCertificate_importBasic
--- PASS: TestAccAWSIAMServerCertificate_importBasic (22.81s)
=== RUN TestAccAWSIAMServerCertificate_basic
--- PASS: TestAccAWSIAMServerCertificate_basic (19.68s)
=== RUN TestAccAWSIAMServerCertificate_name_prefix
--- PASS: TestAccAWSIAMServerCertificate_name_prefix (19.88s)
=== RUN TestAccAWSIAMServerCertificate_disappears
--- PASS: TestAccAWSIAMServerCertificate_disappears (13.94s)
=== RUN TestAccAWSIAMServerCertificate_file
--- PASS: TestAccAWSIAMServerCertificate_file (32.67s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 109.062s
```
Data Source:
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDataSourceIAMServerCertificate_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/25 13:07:10 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSDataSourceIAMServerCertificate_ -timeout 120m
=== RUN TestAccAWSDataSourceIAMServerCertificate_basic
--- PASS: TestAccAWSDataSourceIAMServerCertificate_basic (43.86s)
=== RUN TestAccAWSDataSourceIAMServerCertificate_matchNamePrefix
--- PASS: TestAccAWSDataSourceIAMServerCertificate_matchNamePrefix (2.68s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 46.569s
```
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