resource/aws_cognito_user_pool: Fix perpetual diffs on sms_verificati…

…on_message

When using `verification_message_template.sms_message` for setting the sms message verification template the resource upon subsequent applys will trigger an update because of a diff in the conflicting `sms_verification_message` argument. This changes adds the computed property onto `sms_message_verification` to ensure it get's updated with the contents being set by `verification_message_template.sms_message`

Acceptance test before change
```
--- FAIL: TestAccAWSCognitoUserPool_withVerificationMessageTemplate (10.90s)                                                                                                                                                         [274/322]
    testing.go:568: Step 0 error: After applying this step, the plan was not empty:

        DIFF:

        UPDATE: aws_cognito_user_pool.pool
          sms_verification_message:                                "{####} Baz" => ""

```

Acceptance test after change
```
--- PASS: TestAccAWSCognitoUserPool_withVerificationMessageTemp
```

aws/resource_aws_cognito_user_pool.go

@@ -410,6 +410,7 @@ func resourceAwsCognitoUserPool() *schema.Resource {
 			"sms_verification_message": {
 				Type:          schema.TypeString,
 				Optional:      true,
+				Computed:      true,
 				ValidateFunc:  validateCognitoUserPoolSmsVerificationMessage,
 				ConflictsWith: []string{"verification_message_template.0.sms_message"},
 			},

aws/resource_aws_cognito_user_pool_test.go

@@ -1200,7 +1200,6 @@ resource "aws_cognito_user_pool" "pool" {
 
   email_verification_message = "Foo {####} Bar"
   email_verification_subject = "FooBar {####}"
-  sms_verification_message   = "{####} Baz"
 
   # Setting Verification template attributes like EmailMessage, EmailSubject or SmsMessage
   # will implicitly set EmailVerificationMessage, EmailVerificationSubject and SmsVerificationMessage
@@ -1209,6 +1208,7 @@ resource "aws_cognito_user_pool" "pool" {
     default_email_option  = "CONFIRM_WITH_LINK"
     email_message_by_link = "{##foobar##}"
     email_subject_by_link = "foobar"
+    sms_message           = "{####} Baz"
   }
 }
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