StepModifyAMIAttributes

/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! :))

/cc @jmassara

/cc @jmassara

Conflicts:
	website/source/docs/builders/amazon-instance.html.markdown

Update packer.go to cleanup plugin clients on bad CLI exit. 

Since defer does not fire when exiting the application with os.Exit we need to be sure that we cleanup all of the clients on failures from the CLI and creating a new environment object.

Use explicit name for Tag field and note tags added in the ui.