[CBM] Investigate whether our REST client should fall back to internal addresses when no alternate address is available

What's the issue?
As stated in MB-47393, it's possible not to define an alternate address/port for a given service. By default, if 'cbbackupmgr' finds a zero-valued port, the backup/restore for the service will be skipped (this will be logged).

func handleServiceError(err error, who, what string) bool {

	if rest.IsServiceNotAvailable(err) || rest.IsServiceNotSupported(err) {

		clog.Log("(Couchbase) (%s) Will not be transferring %s because %s", who, what, errutil.Unwrap(err))

		return true

	}

	return false

}

What's the task?
We should investigate whether we should fallback to the internal hostname/port if not alternate hostname/port is available.

If we should be, this change would probably need to be make in 'gocbcore' since it appears to have the same behavior as us in this regard (but I may be wrong).