Plugin for configuring s3cmd and ~/.s3cfg file

This plugin is based heavily on the boto plugin for StarCluster. It creates a .s3cfg file based on the default s3cmd template in the CLUSTER_USER home directory. The plugin can be configured to optionally use gpg and/or https (just as `s3cmd --configure` would allow) and grabs the `aws_access_key_id` and `aws_secret_access_key` from the `master.ec2._conn` object (note: this is a change from the boto.py script-- see my issue regarding this filed separately). 

Default plugin configuration:
```shell
[plugin s3cmd]
setup_class = s3cmd.S3cmdPlugin
```
with optional parameters:
```bash
gpg_command = /path/to/gpg
gpg_passphrase = mypassphrase
use_https = True
```

Future upgrades:
- ability to create this file for all users
- parameterize other s3cmd options
- install newer version of s3cmd (not via apt-get or yum, as these are outdate)