[Autotest] Allow the syncing of labels added/removed.

This allows us to perform label add/remove actions
on the master and have it pass down to the shards those hosts
are on by introducing a generic decorator.

The cl also fixes some logging issues with the shard-client,
and a bug with propogating the invalid bit of a host.

TEST=Added/removed labels.

Change-Id: Ic176054e04a5508bb3e967ecc902422628446d32
Reviewed-on: https://chromium-review.googlesource.com/231558
Reviewed-by: Prashanth B <beeps@chromium.org>
Tested-by: Prashanth B <beeps@chromium.org>
Commit-Queue: Prashanth B <beeps@chromium.org>
7 files changed