bot_update: Add --rietveld_server flag.

scripts/slave/recipes/chromium_trybot.expected/bot_update_failure.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\bot_update.py",
       "--master",
       "tryserver.chromium.win",
       "--builder",
       "win_no_bot_update",
       "--slave",
       "TestSlavename",
       "--spec",
       "cache_dir = None\nsolutions = [{'custom_vars': {'googlecode_url': 'svn://svn-mirror.golo.chromium.org/%s', 'nacl_trunk': 'svn://svn-mirror.golo.chromium.org/native_client/trunk', 'sourceforge_url': 'svn://svn-mirror.golo.chromium.org/%(repo)s', 'webkit_trunk': 'svn://svn-mirror.golo.chromium.org/blink/trunk'}, 'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'svn://svn-mirror.golo.chromium.org/chrome/trunk/src'}]",
       "--root",
       "src",
       "--revision_mapping_file",
       "{\"src\": \"got_revision\", \"src/native_client\": \"got_nacl_revision\", \"src/third_party/WebKit\": \"got_webkit_revision\", \"src/third_party/webrtc\": \"got_webrtc_revision\", \"src/tools/swarming_client\": \"got_swarming_client_revision\", \"src/v8\": \"got_v8_revision\"}",
       "--issue",
       "12853011",
       "--patchset",
       "1",
+      "--rietveld_server",
+      "https://codereview.chromium.org",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@HEAD",
       "--force"
     ],
     "name": "bot_update",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
       "@@@STEP_TEXT@Some step text@@@",
@@ skipping 45 matching lines @@
       "@@@SET_BUILD_PROPERTY@got_swarming_client_revision_git@\"a0ee5d99f1d4b38ddaa4c759c837980b63b99566\"@@@",
       "@@@SET_BUILD_PROPERTY@got_revision_git@\"f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9\"@@@"
     ]
   },
   {
     "name": "$final_result",
     "reason": "Infra Failure: Step('bot_update') returned 1",
     "status_code": 1
   }
 ]