[autotest] Do not create AfeHost in server_job in Skylab.

In Skylab, host information is provided via a prepopulated HostInfoStore
file. In this case, the AFE is not available. So, create a fake AfeHost.

BUG=chromium:851206
TEST=pre-cq

Change-Id: I299148e6b454fec538b33b133f1aa9f329c0197b
Reviewed-on: https://chromium-review.googlesource.com/1097506
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Trybot-Ready: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Xixuan Wu <xixuan@chromium.org>

server/server_job.py

diff --git a/server/server_job.py b/server/server_job.py
index e9774b4..1237505 100644
--- a/server/server_job.py
+++ b/server/server_job.py
@@ -124,14 +124,14 @@
                 afe_host.attributes.update(host_attributes)
                 info = host_info.HostInfo(attributes=host_attributes)
                 host_info_store.commit(info)
-        else:
+        elif use_shadow_store:
             afe_host = _create_afe_host(machine)
-            if use_shadow_store:
-                host_info_store = _create_afe_backed_host_info_store(store_dir,
-                                                                     machine)
-            else:
-                host_info_store = _create_file_backed_host_info_store(store_dir,
-                                                                      machine)
+            host_info_store = _create_afe_backed_host_info_store(store_dir,
+                                                                 machine)
+        else:
+            afe_host = server_utils.EmptyAFEHost()
+            host_info_store = _create_file_backed_host_info_store(store_dir,
+                                                                  machine)
 
         machine_dict_list.append({
                 'hostname' : machine,