[autotest] Hosts locked manually by users should not count as usable.

We need to ignore them in Reimager._count_usable_hosts(), just like we
do 'Repairing' and 'Repair Failed' hosts.

The trick is _not_ painting hosts correctly locked by the test infrastructure
with the same brush.  These hosts will become available in due time.

BUG=chromium-os:33623
TEST=unit
TEST=run_suite while all usable devices are locked manually.  Should fail.
TEST=run_suite while all usable devices are locked for reimaging.  Should run.

Change-Id: I7bf001d9503cdd027741760493cce53947f95e45
Reviewed-on: https://gerrit.chromium.org/gerrit/30768
Commit-Ready: Chris Masone <cmasone@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
Tested-by: Chris Masone <cmasone@chromium.org>
diff --git a/server/cros/dynamic_suite/tools.py b/server/cros/dynamic_suite/tools.py
index 08205ad..8ab674c 100644
--- a/server/cros/dynamic_suite/tools.py
+++ b/server/cros/dynamic_suite/tools.py
@@ -18,6 +18,11 @@
     return _CONFIG.get_config_value('CROS', 'sharding_factor', type=int)
 
 
+def infrastructure_user_list():
+    return _CONFIG.get_config_value('CROS', 'infrastructure_users', type=list,
+                                    default=[])
+
+
 def package_url_pattern():
     return _CONFIG.get_config_value('CROS', 'package_url_pattern', type=str)