img(coreos): Initial commit of CoreOS image build

- Mostly imported from github.com/racker/teeth-agent-image
remove api-url from unit file

- Patch incoming to the agent for it to discover API url (and other
  params) via kernel command line
fix(coreos-oem-inject): Default to dev-channel

- We'll switch this to a 'stable' channel once one exists for coreos
Merge pull request #58 from rackerlabs/jay/IntegrateImageBuilds

img(coreos): Initial commit of CoreOS image build