Configurable Keystone admin workers

The API_WORKERS option wasn't setting the number of admin workers
for the Keystone server when running in eventlet mode. This will
allow for control of throughput and memory usage.

Change-Id: Iecbce4a601f93784164e53d8b0c542d48f7b5650
diff --git a/lib/keystone b/lib/keystone
index be62607..b6a4e10 100644
--- a/lib/keystone
+++ b/lib/keystone
@@ -293,6 +293,9 @@
     fi
 
     iniset $KEYSTONE_CONF DEFAULT max_token_size 16384
+
+    iniset $KEYSTONE_CONF DEFAULT admin_workers "$API_WORKERS"
+    # Public workers will use the server default, typically number of CPU.
 }
 
 function configure_keystone_extensions {