[autotest] Add get_build back to CrosHost class.

CL 323781 removed this method. Add it back here to maintain backwards
compatibility for test control files in older CrOS builds (R48, R49 etc.)
still call host.get_build, e.g., `provision_AutoUpdate.double`.

BUG=None
TEST=local verify, unittest

Change-Id: I6964e3bddb98293e4250965603d989d71e4a2d9b
Reviewed-on: https://chromium-review.googlesource.com/325619
Commit-Ready: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Kevin Cheng <kevcheng@chromium.org>
diff --git a/server/afe_utils.py b/server/afe_utils.py
index 78d7595..d609838 100644
--- a/server/afe_utils.py
+++ b/server/afe_utils.py
@@ -29,11 +29,11 @@
 
 
 def get_build(host):
-    """Retrieve the current build for a given hostname from the AFE.
+    """Retrieve the current build for a given host from the AFE.
 
     Looks through a host's labels in the AFE to determine its build.
 
-    @param hostname: Hostname of the host whose build we want to retrieve.
+    @param host: Host object to get build.
 
     @returns The current build or None if it could not find it or if there
              were multiple build labels assigned to the host.