Add support for CloudFormation Resource Import to CFn glue code

[wjordan]

This update to AWS::CloudFormation makes it possible to import existing resources into a stack using the IMPORT type of the CreateChangeSet API operation.

The implementation defines a new import_resources option to trigger importing resources, derived from the IMPORT_RESOURCES env variable via the stack/adhoc rake tasks.

For example, to import an AWS::EC2::Instance resource with a logical-id of Daemon and an instance id i-0123456789abcdef:

RAILS_ENV=staging bundle exec rake stack:start IMPORT_RESOURCES=Daemon:i-0123456789abcdef

Note the existing limitation of resource imports, which will cause the process to fail if not satisfied:

• An explicit DeletionPolicy must be specified for imported resources.
• The newly-imported resources must be the only change in the updated stack template, changes to any other resources are not permitted.

Since resource import is only possible through creating/executing a Change Set, I also had to change the implementation to apply stack updates through ExecuteChangeSet instead of UpdateStack directly, making this a much more substantial change. I added an extra Proceed? [y/n] confirmation dialog to create_or_update (:start rake tasks), so manually running :validate before running :start is now slightly less crucial to prevent accidental changes from being applied.

[wjordan]

Merging this now in order to unblock other pending work requiring resource imports, will revert if it introduces any new issues in our CI pipeline.