Snippet Resource

1. Snippet
...
resources:
  snippet1:
    type: OS::HEAT::Snippet
    properties:
      inputs:{...}
      outputs: [...] # defaults: stdout, stderr, exit
      snippet: ... # snippet running in heat engine
2. Definition
inputs: key value pairs,
ouputs: name list, the values can be accessed via get_attr,
snippet: a runable script.
    inputs key pairs will be set to environment,
    stdout should be in json format to output,
    exit code is an embedded output
The system will capture the output of the snippet and set it in attributes for get_attr