ipcluster touching ~/.ipython even though --ipython-dir is specified

When trying to run `ipcluster` for testing in a temporary directory, `~/.ipython` still gets created.

```bash
rm -rf ~/.ipython
mkdir /tmp/tmp5aa3_zqu
ipcluster start --profile=testing1 --n=2 --ipython-dir=/tmp/tmp5aa3_zqu
```

```bash
$ tree ~/.ipython/
/home/mforbes/.ipython/
`-- profile_default
|-- db
|-- log
|-- pid
|-- security
`-- startup
`-- README
```

Shouldn't `--ipython-dir` prevent this? (I don't want my tests creating files in someones home directory).

Note, this is very minor: `/tmp/tmp5aa3_zqu` is used for all the important stuff.

Reproduce the issue with the shown `ipcluster start --profile=testing1 --n=2 --ipython-dir=/tmp/tmp5aa3_zqu` command after removing `~/.ipython`. Trace how `--ipython-dir` is handled and identify the startup path that creates `~/.ipython`; done means the command creates no files there while continuing to use the temporary directory.

索引モデルが issue の本文から書いたものです。