Instance store ami plugin

This plugin creates an instance-store based AMI instead of an EBS based one.
The main changes are :
- replaces all the tasks that deal with EBS volumes and use a local loop mounted image,
- install more recent euca2ools (2.1.3) (which requires python-boto >= 2.2, e.g. from squeeze-backports or wheezy)
- require additional AWS authentication information as environment variables (see task 08-more-host-info) and local files (key and cert).