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

recipe_modules/step/api.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import contextlib
 
 from recipe_engine import recipe_api
 
 
 # Inherit from RecipeApiPlain because the only thing which is a step is
@@ skipping 113 matching lines @@
       kwargs['cmd'] = command
 
     kwargs['ok_ret'] = ok_ret
     kwargs['infra_step'] = bool(infra_step)
 
     # Obtain information from composite step parent.
     compositor = recipe_api._STEP_CONTEXT
     name = compositor.get_with_context('name', name)
     kwargs['env'] = compositor.get_with_context('env', kwargs.get('env', {}))
     kwargs['step_nest_level'] = compositor.get_with_context('nest_level', 0)
-    kwargs.setdefault('cwd', self.m.path['slave_build'])
 
     # Disambiguate repeated names
     step_count = self._step_names.setdefault(name, 0) + 1
     self._step_names[name] = step_count
     if step_count > 1:
       name = "%s (%d)" % (name, step_count)
     kwargs['name'] = name
 
     schema = self.make_config()
     schema.set_val(kwargs)
     return self.run_from_dict(self._engine.create_step(schema))
 
   # TODO(martiniss) delete, and make generator_script use **kwargs on step()
   @recipe_api.composite_step
   def run_from_dict(self, dct):
     return self._engine.run_step(dct)