Allows for targeting of the root domain when creating managed dns records

[jimrubenstein]

Implements proposed solution found here #2

[eperry]

I have tested this patch and from that test, this seems to a good solution, and the @ sign is an industry standard. I also tested this with the terraform module for DME and it worked.

I would like to see this solution merged as well

[nkatarmal-crest]

We can fix it without introducing a new placeholder for empty value. I will look into this.