migrate certs into the blobs database

use platformdata/nginx/cert to store the certs

src/blobs.js

@@ -14,6 +14,8 @@ exports = module.exports = {
     SFTP_PUBLIC_KEY: 'sftp_public_key',
     SFTP_PRIVATE_KEY: 'sftp_private_key',
 
+    CERT_PREFIX: 'cert',
+
     _clear: clear
 };