Adds computed `arn` to security group data source
```
$ make testacc TEST=./builtin/providers/aws TESTARGS="-run=TestAccDataSourceAwsSecurityGroup"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/05 10:17:35 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccDataSourceAwsSecurityGroup -timeout 120m
=== RUN TestAccDataSourceAwsSecurityGroup
--- PASS: TestAccDataSourceAwsSecurityGroup (56.72s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 56.725s
```
* provider/aws: data source for AWS Security Group
* provider/aws: add documentation for data source for AWS Security Group
* provider/aws: data source for AWS Security Group (improve if condition and syntax)
* fix fmt
Jake Champlin