Explicitly add user_data plugin to list of default

The plugin would be picked up implicitly, even by default. Still, there
is no harm done in having it listed explicitly, which is more
future-proof.
Fix validation of instance attribute 'shutdown-behavior'

Accepts string values 'stop' and 'terminate'.
Setup logging, so modules can use it

Introduces a --verbose command line option, to expose the verbosity
level to the user. Mostly useful for debugging.
AMI Image Search plugin

This exposes EC2's DescribeImages API call. All filters currently
available in the API are exposed, but not validated. This construct does
not accept queries by ImageId, as that would be rather pointless here.

Wildcard characters '*' and '?' are recognized in filter values. Online
AWS docs have more info:

    http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeImages.html

This allows instance definitions to be (a bit)gq more independent of
specific regions and enables them to effectively track the latest
version of a specific distribution.`