http/tcp status checks should not embed address in definition

[mmlb]

Issue Description

The httpstatus arg has a required url argument, httpstatus checker is passing this straight through to http.NewRequest. This makes no sense since there will likely be > 1 nodes running the service and the ip can't be known at service definition time. See discussion https://gitter.im/cerana/cerana?at=579798fa8fb4820a4665e4ef

Similar situation exists in tcp health checker.

[bbickerton]

For now, define urls with localhost. Once networking is complete, we can try to be more clever and correct.