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>

For the Web Frontend, we require a DB change to make the host
attributes panel visible in the admin interface.

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.
DEPLOY=migrate

Change-Id: I9fb3e8ab6d386104c7e9ed555eaa110a0ae72b02
Reviewed-on: https://chromium-review.googlesource.com/253760
Reviewed-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Simran Basi <sbasi@chromium.org>
Tested-by: Simran Basi <sbasi@chromium.org>
5 files changed