autotest: Randomly distribute devices to multiple skylab_drones.

BUG=chromium:869940
TEST=Run "atest host migrate --model=nyan_blaze --pool=bvt --env=prod
--migration" in migration.

Change-Id: I63ede94d22d23f78d0b92be54c439ca383df7965
Reviewed-on: https://chromium-review.googlesource.com/1163751
Commit-Ready: Xixuan Wu <xixuan@chromium.org>
Tested-by: Xixuan Wu <xixuan@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
diff --git a/cli/host.py b/cli/host.py
index aa1ba03..9f681c4 100644
--- a/cli/host.py
+++ b/cli/host.py
@@ -1246,8 +1246,10 @@
                 'environment. Please confirm there is at least one valid skylab'
                 ' drone added in skylab inventory.')
 
-        skylab_drone = random.choice(skylab_drones)
-        skylab_server.add_dut_uids(skylab_drone, devices)
+        for device in devices:
+            # Randomly distribute each device to a skylab_drone.
+            skylab_drone = random.choice(skylab_drones)
+            skylab_server.add_dut_uids(skylab_drone, [device])
 
 
     def remove_duts_from_drone(self, infra, devices):