recipe engine: extract infra-specific paths out of the engine

recipe_modules/generator_script/example.py

Index: recipe_modules/generator_script/example.py
diff --git a/recipe_modules/generator_script/example.py b/recipe_modules/generator_script/example.py
index 3fd91361988a3ddc901f977e1852c0e6ed120a24..4acee8e7cef61214998ba9b6ac39ef414e220bb3 100644
--- a/recipe_modules/generator_script/example.py
+++ b/recipe_modules/generator_script/example.py
@@ -18,7 +18,7 @@ PROPERTIES = {
 }
 
 def RunSteps(api, script_name, script_env):
-  api.path['checkout'] = api.path['slave_build']
+  api.path['checkout'] = api.path['tmp_base'].join('checkout')

[iannucci, 2016/04/23 20:13:08]

is this a good example? I mean; if someone copy-pasted this recipe, is this
something they'd have to change, or is this actually going to be a recommended
practice? Maybe this should just be relative to the current working directory?

[Paweł Hajdan Jr., 2016/04/26 08:34:15]

Well, it's not worse than before. Nothing is checking things out there anyway.
We discussed this offline.

If you have ideas how to further improve this, please share.

   script_name = api.properties['script_name']
   script_env = api.properties.get('script_env')
   api.generator_script(script_name, env=script_env)