provider/aws: Add support for Skipping Final Snapshot in RDS Cluster

This works exactly the same as in DB Instance. We cannot use an empty
string to skip the final snapshot or we get the following error:

```
only alphanumeric characters and hyphens allowed in
"final_snapshot_identifier"
```

Therefore, we need to wrap this with another parameter. The we DO NOT
skip final snapshots by default as we currently create them right now
and this would change the user functionality

builtin/providers/aws/resource_aws_rds_cluster.go

@@ -103,6 +103,12 @@ func resourceAwsRDSCluster() *schema.Resource {
 				},
 			},
 
+			"skip_final_snapshot": &schema.Schema{
+				Type:     schema.TypeBool,
+				Optional: true,
+				Default:  false,
+			},
+
 			"master_username": &schema.Schema{
 				Type:     schema.TypeString,
 				Required: true,
@@ -372,8 +378,9 @@ func resourceAwsRDSClusterDelete(d *schema.ResourceData, meta interface{}) error
 		DBClusterIdentifier: aws.String(d.Id()),
 	}
 
+	skipFinalSnapshot := d.Get("skip_final_snapshot").(bool)
 	finalSnapshot := d.Get("final_snapshot_identifier").(string)
-	if finalSnapshot == "" {
+	if skipFinalSnapshot == true {
 		deleteOpts.SkipFinalSnapshot = aws.Bool(true)
 	} else {
 		deleteOpts.FinalDBSnapshotIdentifier = aws.String(finalSnapshot)

website/source/docs/providers/aws/r/rds_cluster.html.markdown

@@ -61,6 +61,7 @@ string.
 * `final_snapshot_identifier` - (Optional) The name of your final DB snapshot
     when this DB cluster is deleted. If omitted, no final snapshot will be
     made.
+* `skip_final_snapshot` - (Optional) Determines whether a final DB snapshot is created before the DB cluster is deleted. If true is specified, no DBSnapshot is created. If false is specified, a DB snapshot is created before the DB cluster is deleted. Default is false.
 * `availability_zones` - (Optional) A list of EC2 Availability Zones that
   instances in the DB cluster can be created in
 * `backup_retention_period` - (Optional) The days to retain backups for. Default