Consul TCP Checks Should Default to Bind Addr

Looking at the documentation for service configs, this part has me a bit confused:

For a service definition:

> The address field can be used to specify a service-specific IP address. __By default, the IP address of the agent is used, and this does not need to be provided__. The port field can be used as well to make a service-oriented architecture simpler to configure; this way, the address and port of a service can be discovered.

And for a TCP health check definition:

> TCP + Interval - These checks make an TCP connection attempt every Interval (e.g. every 30 seconds) to the specified IP/hostname and port. __If no hostname is specified, it defaults to "localhost"__.

It seems to me that the service's bind address should be honored as the default for all child healthcheck definitions for that service, instead of switching over to `localhost`. Otherwise, in the current scenario, there is no way to set up a TCP healthcheck for a service that listens __only__ on the bind address without knowing the node's external IP in the first place.

This suggestion may be a backwards incompatible change, but it seems to be more in line with the service-to-health-check relationship.

If this is not an option, then a workaround like `GetInteraceIP` should perhaps be made available in the healthcheck definition as it is through command-line arguments so users can dynamically specify the target IP to a non-loopback address. Making envvars available to be injected similarly might also work, and be more flexible.