Add --buildmetadata to records AMI info

I'm using ec2debian-build-ami to build images via Jenkins.
I would like to use the metadata from the image creation process
to automate other phases of deployment.

--buildmetada adds a shell script file output option containing
variables which can be sourced into other scripts so you can
use the values in an scripted fashion.
Add a filename parameter placeholder
Respect white space don't use tabs.
Add build metadata using a plugin
Use recommended global checking idiom
Merge pull request #60 from yanfalies/master

Add a build metadata output file to record the ami and snapshot ids for further scripting