Adding customizations for ec2 bundle-instance

This PR adds some customizations to ``aws ec2 bundle-instance`` to allow users to enter the required values via simple scalar parameters rather than constructing the complex JSON data structure.  It also generates a default policy if a policy is not provided and signs the policy file for the user if the ``--user-sak`` option is provided.

This PR is dependent upon https://github.com/boto/botocore/pull/99