[autotest] Add tag rw_only to provision_FirmwareUpdate test for RW provision job

This is to make sure provision_FirmwareUpdate for RW only update saves result
to a different folder other than provision_FirmwareUpdate. So a single provision
job can update RO and RW to different build.

BUG=chromium:601861
TEST=local test

Change-Id: Ief8824c394a4d8d62929a5e42534a6c4887ba5b2
Reviewed-on: https://chromium-review.googlesource.com/338040
Commit-Ready: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@google.com>
Reviewed-by: Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: Dan Shi <dshi@google.com>
diff --git a/server/cros/provision.py b/server/cros/provision.py
index 0215f3f..ac2800c 100644
--- a/server/cros/provision.py
+++ b/server/cros/provision.py
@@ -178,7 +178,8 @@
                 'provision_FirmwareUpdate'),
         FW_RW_VERSION_PREFIX: actionables.TestActionable(
                 'provision_FirmwareUpdate',
-                extra_kwargs={'rw_only': True}),
+                extra_kwargs={'rw_only': True,
+                              'tag': 'rw_only'}),
         ANDROID_BUILD_VERSION_PREFIX : actionables.TestActionable(
                 'provision_AndroidUpdate'),
     }