Rework script for syncing bootstrap files with S3.

It now creates the bucket you need for a particular env then uploads any
bootstrap scripts it can find.
User-data now pulls config from S3

This commit alters how the user-data script works. It now requires the
EC2 instance to have a 'bucket' tag which stores the S3 bucket URL. The
user-data script downloads all scripts in the $bucket/bootstrap folder
and executes them. It exports the BUCKET_URL variable to the downloaded
bootstrapping scripts can use it.
Download things quietly in base bootstrap script

And create the /host/ folder.
Use Fabric to interact with S3

This fabfile provides tasks for syncing bootstrap and release files onto
S3.
Improve the "webserver" bootstrap script

The script now fetches the key details from S3 such as:

- the hostnames nginx should respond to
- the docker image to deploy
- the sensitive env variables to pass