Fixes [SPARK-5434](https://issues.apache.org/jira/browse/SPARK-5434).

Simple demonstration of the problem and the fix:

```
$ spacey_path="/path/with some/spaces"
$ dirname $spacey_path
usage: dirname path
$ echo $?
1
$ dirname "$spacey_path"
/path/with some
$ echo $?
0
```

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #4224 from nchammas/patch-1 and squashes the following commits:

960711a [Nicholas Chammas] [EC2] Preserve spaces in EC2 path