Fixed Nginx vhosts test.

tests/test_nginx_vhosts.py

@@ -15,6 +15,7 @@ import sys
 nginx_dir = '/etc/nginx/sites-enabled'
 if not os.path.exists(nginx_dir):
     print('Nginx dir does not exists ("%s").' % nginx_dir)
+    sys.exit(0)
 
 # check that Wowza is installed
 wowza_dir = '/usr/local/WowzaStreamingEngine'