us-east-1 (default region) is not used in the Cloudfront hostname

[cssjoe]

Fixes https://github.com/BoldGrid/w3-total-cache/issues/845

This enhancement also adds a display span for the effective hostname, visible on the settings page.

To test:

• Create or have an existing Cloudfront distribution using an S3 bucket for storage in the "us-east-1" default region.
• Enter the CDN details in the CDN Cloudfront S3 push configuration.
• Use the test button to see that it passes for both US East selections for "bucket".

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 44 lines in your changes are missing coverage. Please review.

Project coverage is 4.22%. Comparing base (f3b20e4) to head (6e177ce).

Files	Patch %	Lines
CdnEngine_CloudFront.php	0.00%	22 Missing :warning:
CdnEngine_S3.php	0.00%	12 Missing :warning:
Cdn_Core.php	0.00%	9 Missing :warning:
inc/options/cdn/cf.php	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #871 +/- ## =========================================== - Coverage 4.22% 4.22% -0.01% - Complexity 21571 21577 +6 =========================================== Files 873 873 Lines 94949 94971 +22 =========================================== Hits 4010 4010 - Misses 90939 90961 +22 ```

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