Added route53domains endpoint to region_config.json

route53domains actions fail if you are configured to use any region other than us-east-1.

It appears to be because there is no global endpoint configured for this service and the local dns entries don't exit (route53domains.eu-west-1.amazonaws.com, etc).

There also isn't an endpoint that matches the pattern the other global endpoints have ({service}.amazonaws.com).

This commit allows route53domains queries to work but I'm not sure it's exactly right - I suspect adding route53domains.amazonaws.com to the dns would be preferable to the SDK hardcoding us-east-1 and I wasn't sure about the https prefix in front of the endpoint (some have it, some don't.)

Thanks,

Daniel