A service without a defined port generates errors

v0.1.1

AWS Cloud Map doesn't seem happy about creating services that have no ports.

Given a minimal service definition of:
```
service {
name = "foo"
}
```

The consul-aws output is spammed with:
```
2019-02-05T20:19:29.085Z [ERROR] aws: cannot create nodes: error="InvalidInput: A valid port number must be supplied.
```

Setting a port of 0 has no effect.