builder/amazon: add clean_ami_name template function

Add a clean_ami_name template function which will translate illegal
characters in an AMI name to '-'. Example usage would be:
    "ami_name": "Ubuntu 12.04 {{isotime | clean_ami_name}}"
website: document isotime and clean_ami_name
website: add no_device to amazon block device doc
Update CHANGELOG to include clean_ami_name