Moblab: Servo Support via Host Attributes.

Moblab devices will look for servo_args inside a host's host
attributes.

Host attributes can be set via the CLI or the Web Frontend.

CLI to see attributes:
atest host stat <host>

CLI to set attribute:
atest host mod --attribute <attribute> --value <value> <host>

Updated the afe's management system so that it properly configures
the admin interface to allow editing of host attributes.

BUG=chromium:394544
TEST=local moblab setup. Tested both CLI and AFE host attribute
manipulation. Launched Servod, and ensure servo host is only
created when the attribute is applied to the host.

Change-Id: Ie3cccab31aa7518435ef0abc6ce206363406c272
Reviewed-on: https://chromium-review.googlesource.com/255550
Reviewed-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Simran Basi <sbasi@chromium.org>
Tested-by: Simran Basi <sbasi@chromium.org>
diff --git a/frontend/afe/management.py b/frontend/afe/management.py
index 70fd775..3e063ad 100644
--- a/frontend/afe/management.py
+++ b/frontend/afe/management.py
@@ -25,7 +25,7 @@
     PermissionModel = auth.models.Permission
     have_permissions = list(admin_group.permissions.all())
     for model_name in ('host', 'label', 'test', 'aclgroup', 'profiler',
-                       'atomicgroup'):
+                       'atomicgroup', 'hostattribute'):
         for permission_type in ('add', 'change', 'delete'):
             codename = permission_type + '_' + model_name
             permissions = list(PermissionModel.objects.filter(