Fix NOVA_API_VERSION option

lib/nova checks NOVA_API_VERSION for setting Nova v2.1 API endpoint,
but the name of the initialization is different.
This patch fixes it.

Change-Id: I2fa234d66fe76b2a7146288adc1384b33921e178
diff --git a/lib/nova b/lib/nova
index 5db2f8f..ec2d0bf 100644
--- a/lib/nova
+++ b/lib/nova
@@ -45,12 +45,12 @@
 NOVA_CELLS_DB=${NOVA_CELLS_DB:-nova_cell}
 
 NOVA_API_PASTE_INI=${NOVA_API_PASTE_INI:-$NOVA_CONF_DIR/api-paste.ini}
-# NOVA_API_VERSIONS valid options
+# NOVA_API_VERSION valid options
 #   - default - setup API end points as nova does out of the box
 #   - v21default - make v21 the default on /v2
 # NOTE(sdague): this is for transitional testing of the Nova v21 API.
 # Expect to remove in L or M.
-NOVA_API_VERSIONS=${NOVA_API_VERSIONS:-default}
+NOVA_API_VERSION=${NOVA_API_VERSION-default}
 
 if is_ssl_enabled_service "nova" || is_service_enabled tls-proxy; then
     NOVA_SERVICE_PROTOCOL="https"