resource/aws_opsworks_stack: Switch custom Opsworks client User-Agent…

… to real Terraform version

Reference: hashicorp/terraform#13024
Reference: #9570

This migration is required for migrating to the standalone Terraform Plugin SDK and removing our dependency on github.com/hashicorp/terraform in the future.

Output from acceptance testing:

```
--- PASS: TestAccAWSOpsWorksStack_classic_endpoints (28.89s)
```

User-Agent from testing above:

```
User-Agent: aws-sdk-go/1.24.4 (go1.13; darwin; amd64) APN/1.0 HashiCorp/1.0 Terraform/0.12.7
```

aws/config.go

@@ -295,6 +295,7 @@ type AWSClient struct {
 	stsconn                             *sts.STS
 	supportedplatforms                  []string
 	swfconn                             *swf.SWF
+	terraformVersion                    string
 	transferconn                        *transfer.Transfer
 	wafconn                             *waf.WAF
 	wafregionalconn                     *wafregional.WAFRegional
@@ -481,6 +482,7 @@ func (c *Config) Client() (interface{}, error) {
 		storagegatewayconn:                  storagegateway.New(sess.Copy(&aws.Config{Endpoint: aws.String(c.Endpoints["storagegateway"])})),
 		stsconn:                             sts.New(sess.Copy(&aws.Config{Endpoint: aws.String(c.Endpoints["sts"])})),
 		swfconn:                             swf.New(sess.Copy(&aws.Config{Endpoint: aws.String(c.Endpoints["swf"])})),
+		terraformVersion:                    c.terraformVersion,
 		transferconn:                        transfer.New(sess.Copy(&aws.Config{Endpoint: aws.String(c.Endpoints["transfer"])})),
 		wafconn:                             waf.New(sess.Copy(&aws.Config{Endpoint: aws.String(c.Endpoints["waf"])})),
 		wafregionalconn:                     wafregional.New(sess.Copy(&aws.Config{Endpoint: aws.String(c.Endpoints["wafregional"])})),

aws/resource_aws_opsworks_stack.go

@@ -7,7 +7,6 @@ import (
 
 	"github.com/hashicorp/terraform/helper/resource"
 	"github.com/hashicorp/terraform/helper/schema"
-	"github.com/hashicorp/terraform/terraform"
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/arn"
@@ -387,7 +386,7 @@ func opsworksConnForRegion(region string, meta interface{}) (*opsworks.OpsWorks,
 		return nil, fmt.Errorf("Error creating AWS session: %s", err)
 	}
 
-	sess.Handlers.Build.PushBack(request.MakeAddToUserAgentHandler("APN/1.0 HashiCorp/1.0 Terraform", terraform.VersionString()))
+	sess.Handlers.Build.PushBack(request.MakeAddToUserAgentHandler("APN/1.0 HashiCorp/1.0 Terraform", meta.(*AWSClient).terraformVersion))
 
 	newSession := sess.Copy(&aws.Config{Region: aws.String(region)})
 	newOpsworksconn := opsworks.New(newSession)