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.

website: add documentation on installing through Homebrew

provisioner/shell,file: use common for config decoding

Refactor the EBS ami tag into a common step and add support
for instance-store ami tags.
/cc @jmassara

builder/amazon/ebs: Added tagging support for amazon/ebs AMIs

builder/amazon/common: fix check for vpcid