Taupage expiration based on tag

[harti2006]

At the moment, the expiration date of Taupage AMIs (the date after which OUTDATED_TAUPAGE violations are raised) is calculated based on the build timestamp which is encoded in the image name.

Expected behavior

In order to allow more flexible AMI management, e.g. when it comes to important security updates, the expiration time should just be read from tag called ExpirationTime. The value of this tag needs to follow the "date-time" format as defined in RFC-3339. A missing tag simply means that there is no expiration time set, and hence the image is valid indefinitely.

Dependencies

Tags are only available in the account where the image has been built.