bot_update: Add --rietveld_server flag.

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_mac.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "tryserver.webrtc",
       "--builder",
       "mac",
       "--slave",
       "slavename",
       "--spec",
       "cache_dir = None\nsolutions = [{'custom_vars': {'root_dir': 'src'}, 'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'svn://svn-mirror.golo.chromium.org/webrtc/trunk'}, {'custom_vars': {'root_dir': 'src'}, 'deps_file': 'DEPS', 'managed': True, 'name': 'webrtc-limited', 'url': 'svn://svn-mirror.golo.chromium.org/chrome-internal/trunk/webrtc-limited'}]",
       "--revision_mapping_file",
       "{\"src\": \"got_revision\"}",
       "--issue",
       "666666",
       "--patchset",
       "1",
+      "--rietveld_server",
+      "https://fake.rietveld.url",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@12345"
     ],
     "name": "bot_update",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"did_run\": false, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_failure\": false@@@",
@@ skipping 427 matching lines @@
       "{\"BUILD_CONFIG\": \"Debug\", \"buildername\": \"mac\", \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--annotate=gtest",
       "--test-type=voice_engine_unittests",
       "--builder-name=mac",
       "--slave-name=slavename",
       "voice_engine_unittests"
     ],
     "name": "voice_engine_unittests"
   }
 ]