autotest: Uniformly support DUT labels in status_history

So far, get_multiple_histories supported the 'board' and 'pool'
arguments. We now need 'model'. Just support generic 'labels' instead.

BUG=chromium:780892
TEST=(1) unittests (didn't catch bugs I had in earlier iterations)
     (2) ./site_utils/balance_pools.py -n -t 3 cq link
     (3) ./site_utils/dut_status.py -b link -p cq
     (4) grep for and update all callers of get_multiple_histories.

Change-Id: I57cc2e0e59e9532d4fd76aa1718d97787eda0a6e
Reviewed-on: https://chromium-review.googlesource.com/762098
Commit-Ready: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/utils/labellib.py b/utils/labellib.py
index 8a71da4..797cd90 100644
--- a/utils/labellib.py
+++ b/utils/labellib.py
@@ -52,7 +52,7 @@
     string labels.
     """
 
-    def __init__(self, str_labels):
+    def __init__(self, str_labels=()):
         self._plain_labels = []
         self._keyval_map = collections.OrderedDict()
         for str_label in str_labels: