Config's control_listener field is an Option<TapSettings> as of linkerd/linkerd2-proxy#295. It would be more accurate to call this field tap_settings now that if some, it holds both the listener and tap_svc_name.

This change touches a few places of the codebase that the original PR did not--hence the separate issue.

It may be also worth to consider changing the naming of LINKERD2_PROXY_CONTROL_LISTEN_ADDR since this is also only used by tap.