provider/aws: Modifying the parameter_group_name of

aws_elasticache_replication_group caused a panic

Fixes #9097

The update for `parameter_group_name` was trying to find the incorrect
value to set `cache_parameter_group_name` - this is what was causing the
panic

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSElasticacheReplicationGroup_updateParameterGroup'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/09/28 11:17:30 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSElasticacheReplicationGroup_updateParameterGroup -timeout
120m
=== RUN   TestAccAWSElasticacheReplicationGroup_updateParameterGroup
--- PASS: TestAccAWSElasticacheReplicationGroup_updateParameterGroup
(903.90s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws903.931s
```

builtin/providers/aws/resource_aws_elasticache_replication_group.go

@@ -275,7 +275,7 @@ func resourceAwsElasticacheReplicationGroupUpdate(d *schema.ResourceData, meta i
 	}
 
 	if d.HasChange("parameter_group_name") {
-		params.CacheParameterGroupName = aws.String(d.Get("cache_parameter_group_name").(string))
+		params.CacheParameterGroupName = aws.String(d.Get("parameter_group_name").(string))
 		requestUpdate = true
 	}
 

builtin/providers/aws/resource_aws_elasticache_replication_group_test.go

@@ -98,6 +98,36 @@ func TestAccAWSElasticacheReplicationGroup_updateNodeSize(t *testing.T) {
 	})
 }
 
+//This is a test to prove that we panic we get in https://github.com/hashicorp/terraform/issues/9097
+func TestAccAWSElasticacheReplicationGroup_updateParameterGroup(t *testing.T) {
+	var rg elasticache.ReplicationGroup
+	rName := acctest.RandString(10)
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSElasticacheReplicationDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccAWSElasticacheReplicationGroupConfig(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSElasticacheReplicationGroupExists("aws_elasticache_replication_group.bar", &rg),
+					resource.TestCheckResourceAttr(
+						"aws_elasticache_replication_group.bar", "parameter_group_name", "default.redis2.8"),
+				),
+			},
+
+			resource.TestStep{
+				Config: testAccAWSElasticacheReplicationGroupConfigUpdatedParameterGroup(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSElasticacheReplicationGroupExists("aws_elasticache_replication_group.bar", &rg),
+					resource.TestCheckResourceAttr(
+						"aws_elasticache_replication_group.bar", "parameter_group_name", "allkeys-lru"),
+				),
+			},
+		},
+	})
+}
+
 func TestAccAWSElasticacheReplicationGroup_vpc(t *testing.T) {
 	var rg elasticache.ReplicationGroup
 	resource.Test(t, resource.TestCase{
@@ -293,6 +323,50 @@ resource "aws_elasticache_replication_group" "bar" {
 }`, rName, rName, rName)
 }
 
+func testAccAWSElasticacheReplicationGroupConfigUpdatedParameterGroup(rName string) string {
+	return fmt.Sprintf(`
+provider "aws" {
+  region = "us-east-1"
+}
+resource "aws_security_group" "bar" {
+    name = "tf-test-security-group-%s"
+    description = "tf-test-security-group-descr"
+    ingress {
+        from_port = -1
+        to_port = -1
+        protocol = "icmp"
+        cidr_blocks = ["0.0.0.0/0"]
+    }
+}
+
+resource "aws_elasticache_security_group" "bar" {
+    name = "tf-test-security-group-%s"
+    description = "tf-test-security-group-descr"
+    security_group_names = ["${aws_security_group.bar.name}"]
+}
+
+resource "aws_elasticache_parameter_group" "bar" {
+    name = "allkeys-lru"
+    family = "redis2.8"
+
+    parameter {
+        name = "maxmemory-policy"
+        value = "allkeys-lru"
+    }
+}
+
+resource "aws_elasticache_replication_group" "bar" {
+    replication_group_id = "tf-%s"
+    replication_group_description = "test description"
+    node_type = "cache.m1.small"
+    number_cache_clusters = 2
+    port = 6379
+    parameter_group_name = "${aws_elasticache_parameter_group.bar.name}"
+    security_group_names = ["${aws_elasticache_security_group.bar.name}"]
+    apply_immediately = true
+}`, rName, rName, rName)
+}
+
 func testAccAWSElasticacheReplicationGroupConfigUpdatedDescription(rName string) string {
 	return fmt.Sprintf(`
 provider "aws" {