Implement missing HTTP host to ConsulResolver func for Connect SDK.

[banks]

I managed to remove a TODO in Connect SDK that was not done. I thought it was because we have tests for HTTPClient but the part that we made pluggable to ease testing actually didn't have a non-test implementation 😱 .

This fixes that.

UX Gotcha

In general this will work OK as a basic experience. It's a bit of a gotcha that we only support .consul TLD in the HTTP client hostnames even if your actual Consul DNS is configured with a custom domain. The reason for that is that without making external calls and adding much complication, we don't actually know what the correct domain is, and in conjunction with the datacenter suffix being optional, it becomes impossible to unambiguously tell the difference between a custom domain segment and a datacenter name (which matters because we want to support explicit requests to other DCs).

We could potentially revisit this in the future if it seems to be a big issue for folks but it is simplest to just document it for now.

Docs updates to explain this:

[mitchellh]

All looks good to me.