New plugin that creates instance-store based AMIs

This plugins allows the creation of instance-store based AMIs instead
of EBS based. It requires having a ~/.eucarc file with auth infos
required for S3 access.
Replace euca2tools task to install v2.1.3

The euca2tools script used by this plugin to handle bundles have problems in
version 1.x so we need to upgrade. Unfortunately, versions 2.x also need a
version of boto >= 2.2 (which can be installed from squeeze-backports).
Add task that sets more env vars for euca2ools

This avoids having to create a .euca2ools file and also checks that
all information was provided before starting the build process.
Add task to delete bundled image files after upload
Readd and update comment on register failure
Improve bundling tasks

- Use envvar for bucket name
- use $ami_name as bundle prefix