Add simple depends_on API.

recipes/engine_tests/depend_on/top.py

+from recipe_engine.recipe_api import Property
+from recipe_engine import config
+
+DEPS = [
+    'recipe_engine/properties',
+]
+
+PROPERTIES = {
+    'to_pass': Property(),
+}
+
+RETURN_SCHEMA = config.ReturnSchema(
+    result=config.Single(int),
+)
+
+def RunSteps(api, to_pass):
+  res = api.depend_on('engine_tests/depend_on/bottom', {'number': to_pass})
+  return RETURN_SCHEMA(result=res['number_times_two'])
+
+def GenTests(api):
+  yield (
+      api.test('basic') +
+      api.properties(to_pass=3) +
+      api.depend_on('engine_tests/depend_on/bottom', {'number': 3}, {'number_times_two': 6})
+  )