* 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
```