swarming: always append task's OS dimension to step name

scripts/slave/recipe_modules/swarming/example.expected/basic_0.4.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::git]/resources/git_setup.py",
       "--path",
       "[SLAVE_BUILD]/swarming.client",
       "--url",
       "https://chromium.googlesource.com/external/swarming.client.git"
@@ skipping 254 matching lines @@
       "data:hash_for_linux",
       "--tag",
       "master:tryserver",
       "--tag",
       "name:hello_world",
       "--tag",
       "os:Ubuntu-12.04",
       "--tag",
       "os:linux",
       "--tag",
-      "stepname:hello_world",
+      "stepname:hello_world on Ubuntu-12.04",
       "--verbose",
       "--idempotent",
       "--user",
       "joe",
       "hash_for_linux"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "[trigger] hello_world",
+    "name": "[trigger] hello_world on Ubuntu-12.04",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"hello_world/Ubuntu-12.04/hash_for_l\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"hello_world/Ubuntu-12.04/hash_for_l:2:0\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"shard_index\": 0, @@@",
       "@@@STEP_LOG_LINE@json.output@      \"task_id\": \"10000\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"view_url\": \"https://chromium-swarm-dev.appspot.com/user/task/10000\"@@@",
       "@@@STEP_LOG_LINE@json.output@    }, @@@",
       "@@@STEP_LOG_LINE@json.output@    \"hello_world/Ubuntu-12.04/hash_for_l:2:1\": {@@@",
@@ skipping 161 matching lines @@
       "--verbose",
       "--shards",
       "2",
       "hello_world/Ubuntu-12.04/hash_for_l",
       "--task-summary-json",
       "/path/to/tmp/json",
       "--task-output-dir",
       "[TMP_BASE]/hello_isolated_world_tmp_1/task_output_dir"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "hello_world",
+    "name": "hello_world on Ubuntu-12.04",
     "~followup_annotations": [
       "@@@STEP_TEXT@swarming pending 71s@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"shards\": [@@@",
       "@@@STEP_LOG_LINE@json.output@    {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"abandoned_ts\": null, @@@",
       "@@@STEP_LOG_LINE@json.output@      \"bot_id\": \"vm30\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"completed_ts\": \"2014-09-25T01:42:00.123\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"created_ts\": \"2014-09-25T01:41:00.123\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"durations\": [@@@",
@@ skipping 137 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@  chromium_utils.RemoveDirectory(sys.argv[1])@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]