[autotest] Use labellib for provision.py label constants

BUG=chromium:707999
TEST=Run unit tests

Change-Id: Ifaa577526ba5a8f2262fe344b60eaab0ac941289
Reviewed-on: https://chromium-review.googlesource.com/446877
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
diff --git a/server/cros/provision.py b/server/cros/provision.py
index 2a09b17..7f23939 100644
--- a/server/cros/provision.py
+++ b/server/cros/provision.py
@@ -8,14 +8,16 @@
 
 import common
 from autotest_lib.server.cros import provision_actionables as actionables
+from autotest_lib.utils import labellib
+from autotest_lib.utils.labellib import Key
 
 
 ### Constants for label prefixes
-CROS_VERSION_PREFIX = 'cros-version'
-ANDROID_BUILD_VERSION_PREFIX = 'ab-version'
-TESTBED_BUILD_VERSION_PREFIX = 'testbed-version'
-FW_RW_VERSION_PREFIX = 'fwrw-version'
-FW_RO_VERSION_PREFIX = 'fwro-version'
+CROS_VERSION_PREFIX = Key.CROS_VERSION
+ANDROID_BUILD_VERSION_PREFIX = Key.ANDROID_BUILD_VERSION
+TESTBED_BUILD_VERSION_PREFIX = Key.TESTBED_VERSION
+FW_RW_VERSION_PREFIX = Key.FIRMWARE_RW_VERSION
+FW_RO_VERSION_PREFIX = Key.FIRMWARE_RO_VERSION
 
 _ANDROID_BUILD_REGEX = r'.+/.+/P?([0-9]+|LATEST)'
 _ANDROID_TESTBED_BUILD_REGEX = _ANDROID_BUILD_REGEX + '(,|(#[0-9]+))'