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

/cc @rgarcia

/cc @rgarcia - "Message" in Ui is the non-bold colored messages and
are meant for sub-messages or steps part of a larger message. "Say"
is used for the bold stuff. This is more in line wit how Packer works.

/cc @rgarcia - Not sure why this was capitalized to begin with