[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/global_config.ini b/global_config.ini
index dcfc703..b6da726 100644
--- a/global_config.ini
+++ b/global_config.ini
@@ -137,6 +137,7 @@
 dev_server: http://172.22.50.2:8082,http://172.22.50.205:8082
 crash_server: http://172.22.50.2:8082
 sharding_factor: 1
+infrastructure_users: chromeos-test
 
 image_url_pattern: %s/update/%s
 package_url_pattern: %s/static/archive/%s/autotest/packages