resource/aws_cloudfront_distribution: Switch CallerReference from tim…

…e.Now() to resource.UniqueId()

Reference: #9434

Output from acceptance testing:

```
--- PASS: TestAccAWSCloudFrontDistribution_Origin_EmptyOriginID (4.95s)
--- PASS: TestAccAWSCloudFrontDistribution_Origin_EmptyDomainName (5.08s)
--- PASS: TestAccAWSCloudFrontDistribution_OrderedCacheBehavior_ForwardedValues_Headers (1567.50s)
--- PASS: TestAccAWSCloudFrontDistribution_OrderedCacheBehavior_ForwardedValues_Cookies_WhitelistedNames (1569.28s)
--- PASS: TestAccAWSCloudFrontDistribution_disappears (1574.60s)
--- PASS: TestAccAWSCloudFrontDistribution_DefaultCacheBehavior_ForwardedValues_Headers (1575.19s)
--- PASS: TestAccAWSCloudFrontDistribution_DefaultCacheBehavior_ForwardedValues_Cookies_WhitelistedNames (3398.76s)
--- PASS: TestAccAWSCloudFrontDistribution_orderedCacheBehavior (3567.88s)
--- PASS: TestAccAWSCloudFrontDistribution_OriginGroups (3562.87s)
--- PASS: TestAccAWSCloudFrontDistribution_IsIPV6EnabledConfig (3570.24s)
--- PASS: TestAccAWSCloudFrontDistribution_RetainOnDelete (3571.87s)
--- PASS: TestAccAWSCloudFrontDistribution_noOptionalItemsConfig (3575.29s)
--- PASS: TestAccAWSCloudFrontDistribution_WaitForDeployment (3571.28s)
--- PASS: TestAccAWSCloudFrontDistribution_noCustomErrorResponseConfig (3577.28s)
--- PASS: TestAccAWSCloudFrontDistribution_ViewerCertificate_AcmCertificateArn_ConflictsWithCloudFrontDefaultCertificate (3581.27s)
--- PASS: TestAccAWSCloudFrontDistribution_ViewerCertificate_AcmCertificateArn (3584.68s)
--- PASS: TestAccAWSCloudFrontDistribution_customOrigin (4291.01s)
--- PASS: TestAccAWSCloudFrontDistribution_HTTP11Config (4294.31s)
--- PASS: TestAccAWSCloudFrontDistribution_S3Origin (4368.65s)
--- PASS: TestAccAWSCloudFrontDistribution_multiOrigin (4371.62s)
--- PASS: TestAccAWSCloudFrontDistribution_Enabled (4464.73s)
--- PASS: TestAccAWSCloudFrontDistribution_S3OriginWithTags (4466.44s)
```

aws/cloudfront_distribution_configuration_structure.go

@@ -11,12 +11,12 @@ import (
 	"bytes"
 	"fmt"
 	"strconv"
-	"time"
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/service/cloudfront"
 	"github.com/hashicorp/terraform/flatmap"
 	"github.com/hashicorp/terraform/helper/hashcode"
+	"github.com/hashicorp/terraform/helper/resource"
 	"github.com/hashicorp/terraform/helper/schema"
 )
 
@@ -33,7 +33,7 @@ const cloudFrontRoute53ZoneID = "Z2FDTNDATAQYW2"
 func expandDistributionConfig(d *schema.ResourceData) *cloudfront.DistributionConfig {
 	distributionConfig := &cloudfront.DistributionConfig{
 		CacheBehaviors:       expandCacheBehaviors(d.Get("ordered_cache_behavior").([]interface{})),
-		CallerReference:      aws.String(time.Now().Format(time.RFC3339Nano)),
+		CallerReference:      aws.String(resource.UniqueId()),
 		Comment:              aws.String(d.Get("comment").(string)),
 		CustomErrorResponses: expandCustomErrorResponses(d.Get("custom_error_response").(*schema.Set)),
 		DefaultCacheBehavior: expandCloudFrontDefaultCacheBehavior(d.Get("default_cache_behavior").([]interface{})[0].(map[string]interface{})),