[autotest] Break an import cycle in lab_inventory.

An upcoming change adds a dependency in server.hosts on the
MANAGED_POOLS list.  That dependency introduces a cycle:
  hosts package -> lab_inventory -> servo_host -> hosts package

To break the cycle, this moves the MANAGED_POOLS list into
suite_scheduler.constants.

BUG=None
TEST=unit tests; dry run of lab_inventory script

Change-Id: I502d57631705f5f7dd69c805c2a67ff88b5bc73f
Reviewed-on: https://chromium-review.googlesource.com/419115
Commit-Ready: Richard Barnette <jrbarnette@google.com>
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/site_utils/suite_scheduler/constants.py b/site_utils/suite_scheduler/constants.py
index 3a80bf0..51a31c5 100644
--- a/site_utils/suite_scheduler/constants.py
+++ b/site_utils/suite_scheduler/constants.py
@@ -7,16 +7,46 @@
 
 
 class Labels:
-    """Constants related to label names.
+    """
+    Constants related to label names.
 
     @var BOARD_PREFIX The string with which board labels are prefixed.
-    @var POOL_PREFIX The stright with which pool labels are prefixed."""
+    @var POOL_PREFIX The stright with which pool labels are prefixed.
+    """
     BOARD_PREFIX = 'board:'
     POOL_PREFIX = 'pool:'
 
 
+class Pools:
+    """
+    Well-known pool names used in automated inventory management.
+
+    These are general purpose pools of DUTs that are considered
+    identical for purposes of testing.  That is, a device in one of
+    these pools can be shifted to another pool at will for purposes
+    of supplying test demand.
+
+    Devices in these pools are not allowed to have special-purpose
+    attachments, or to be part of in any kind of custom fixture.
+    Devices in these pools are also required to reside in areas
+    managed by the Platforms team (i.e. at the time of this writing,
+    only in "Atlantis" or "Destiny").
+
+    CRITICAL_POOLS - Pools that must be kept fully supplied in order
+        to guarantee timely completion of tests from builders.
+    SPARE_POOL - A low priority pool that is allowed to provide
+        spares to replace broken devices in the critical pools.
+    MANAGED_POOLS - The set of all the general purpose pools
+        monitored for health.
+    """
+    CRITICAL_POOLS = ['bvt', 'cq', 'continuous', 'cts']
+    SPARE_POOL = 'suites'
+    MANAGED_POOLS = CRITICAL_POOLS + [SPARE_POOL]
+
+
 class Builds:
-    """Constants related to build type.
+    """
+    Constants related to build type.
 
     @var FIRMWARE_RW: The string indicating the given build is used to update
                       RW firmware.