Add method to get the chromeos release version.

BUG=chromium:374981
TEST=Used the api and appended result to a filename. Checked that filename
contained the expected build number.

Change-Id: I10446b6d050a14d59f77b1eb793a1fc0d9bc9ade
Reviewed-on: https://chromium-review.googlesource.com/202774
Reviewed-by: Alex Miller <milleral@chromium.org>
Reviewed-by: Rohit Makasana <rohitbm@chromium.org>
Commit-Queue: Rohit Makasana <rohitbm@chromium.org>
Tested-by: Rohit Makasana <rohitbm@chromium.org>
diff --git a/client/common_lib/site_utils.py b/client/common_lib/site_utils.py
index 54b77a3..2619410 100644
--- a/client/common_lib/site_utils.py
+++ b/client/common_lib/site_utils.py
@@ -129,6 +129,13 @@
     return _lsbrelease_search(r'^CHROMEOS_RELEASE_BOARD=(.+)$', group_id=1)
 
 
+def get_chromeos_release_version():
+    """
+    @return chromeos version in device under test as string. None on fail.
+    """
+    return _lsbrelease_search(r'^CHROMEOS_RELEASE_VERSION=(.+)$', group_id=1)
+
+
 def is_moblab():
     """Return if we are running on a Moblab system or not.