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

check for VpcId was incorrectly checking if VPC is empty, instead of not empty.

/cc @jasonberanek - Just adding this as well because I see this being
common as well.

common: Fix URL and Relative Windows file path support #235/#239

/cc @rgarcia Should just pass it in as a param