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

recipe_modules/properties/example.expected/buildbot_git_scheduled.json

 [
   {
     "cmd": [
       "echo",
       "None"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "echo"
   },
   {
     "cmd": [
       "echo",
       "[('blamelist', ['cool_dev1337@chromium.org', 'hax@chromium.org']), ('branch', 'master'), ('buildbotURL', 'http://c.org/p/cr/'), ('buildername', 'TestBuilder'), ('buildnumber', 571), ('mastername', 'chromium.testing.master'), ('project', ''), ('recipe', 'properties:example'), ('repository', 'https://chromium.googlesource.com/chromium/src.git'), ('revision', 'c14d891d44f0afff64e56ed7c9702df1d807b1ee'), ('slavename', 'TestSlavename'), ('test_prop', None), ('workdir', '/path/to/workdir/TestSlavename')]"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "echo all"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]