[SPARK-4137] [EC2] Don't change working dir on user

This issue was uncovered after [this discussion](https://issues.apache.org/jira/browse/SPARK-3398?focusedCommentId=14187471&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14187471).

Don't change the working directory on the user. This breaks relative paths the user may pass in, e.g., for the SSH identity file.

```
./ec2/spark-ec2 -i ../my.pem
```

This patch will preserve the user's current working directory and allow calls like the one above to work.

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

Closes #2988 from nchammas/spark-ec2-cwd and squashes the following commits:

f3850b5 [Nicholas Chammas] pep8 fix
fbc20c7 [Nicholas Chammas] revert to old commenting style
752f958 [Nicholas Chammas] specify deploy.generic path absolutely
bcdf6a5 [Nicholas Chammas] fix typo
77871a2 [Nicholas Chammas] add clarifying comment
ce071fc [Nicholas Chammas] don't change working dir