Right now we have a method to specifically download boto. This PR generalizes it so it's easy to download additional libraries if we want.

For example, adding new external libraries for spark-ec2 is now as simple as:

```python
external_libs = [
    {
         "name": "boto",
         "version": "2.34.0",
         "md5": "5556223d2d0cc4d06dd4829e671dcecd"
    },
    {
        "name": "PyYAML",
        "version": "3.11",
        "md5": "f50e08ef0fe55178479d3a618efe21db"
    },
    {
        "name": "argparse",
        "version": "1.3.0",
        "md5": "9bcf7f612190885c8c85e30ba41db3c7"
    }
]
```
Likely use cases:
* Downloading PyYAML to allow spark-ec2 configs to be persisted as a YAML file. ([SPARK-925](https://issues.apache.org/jira/browse/SPARK-925))
* Downloading argparse to clean up / modernize our option parsing.

First run output, with PyYAML and argparse added just for demonstration purposes:

```shell
$ ./spark-ec2 --version
Downloading external libraries that spark-ec2 needs from PyPI to /path/to/spark/ec2/lib...
This should be a one-time operation.
 - Downloading boto...
 - Finished downloading boto.
 - Downloading PyYAML...
 - Finished downloading PyYAML.
 - Downloading argparse...
 - Finished downloading argparse.
spark-ec2 1.2.1
```

Output thereafter:

```shell
$ ./spark-ec2 --version
spark-ec2 1.2.1
```

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

Closes #4919 from nchammas/setup-ec2-libs and squashes the following commits:

a077955 [Nicholas Chammas] print default region
c95fb7d [Nicholas Chammas] to docstring
5448845 [Nicholas Chammas] remove libs added for demo purposes
60d8c23 [Nicholas Chammas] generalize ability to download libs