builder/amazon/common: use new API for goamz to set AMI attributes

/cc @jmassara
builder/amazon/common: on modify AMI attributes, skip if no work

/cc @jmassara
builder/amazon/common: AMIConfig common config

/cc @jmassara - I pulled out the AMI stuff into a common config
struct and also added in the new template stuff (that didn't exist
when you made the pull, so not your fault! :))
builder/amazon/common: rename StepModifyAttributes to
StepModifyAMIAttributes
builder/amazon/chroot: can modify AMI attributes
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