[autotest] In status_history, convert some methods to properties.

This converts HostJobHistory.get_host_board() and get_host_pool()
to @property methods.  Also add a new host property.

BUG=None
TEST=run unit tests

Change-Id: I42f8d0792ee7f83655581225584e86ea3065cd08
Reviewed-on: https://chromium-review.googlesource.com/268350
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Richard Barnette <jrbarnette@chromium.org>
diff --git a/site_utils/lab_inventory.py b/site_utils/lab_inventory.py
index 182a62f..a50e95b 100755
--- a/site_utils/lab_inventory.py
+++ b/site_utils/lab_inventory.py
@@ -190,7 +190,7 @@
                             remembered.
 
         """
-        pool = host_history.get_host_pool()
+        pool = host_history.host_pool
         self._pools[pool].record_host(host_history)
 
 
@@ -292,12 +292,12 @@
 
 
     def __init__(self, histories):
-        boards = set([h.get_host_board() for h in histories])
+        boards = set([h.host_board for h in histories])
         initval = { board: _BoardCounts() for board in boards }
         super(_LabInventory, self).__init__(initval)
         self._dut_count = len(histories)
         for h in histories:
-            self[h.get_host_board()].record_host(h)
+            self[h.host_board].record_host(h)
 
 
     def get_num_duts(self):