Consider --port option for `jpi server`.

* Consider `--port` option for `jpi server`.
* When `--httpPort` is specified in `JENKINS_OPTS`, `--port` option is ignored.

```
$ jpi server --port=8081
...
INFO: HTTP Listener started: port=8081
...
```

```
$ JENKINS_OPTS="--httpPort=8082" jpi server
...
INFO: HTTP Listener started: port=8082
...
```

```
$ JENKINS_OPTS="--httpPort=8082" jpi server --port=8081
...
INFO: HTTP Listener started: port=8082
...
```