Fix flake8 issues related to the use of global

Change-Id: I65ff4d136a5a7ca324d0cda6bba9063dd4d1b99d
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>

osm_nbi/nbi.py

@@ -1931,7 +1931,6 @@ def _start_service():
     Set database, storage, message configuration
     Init database with admin/admin user password
     """
-    global nbi_server
     global subscription_thread
     global cef_logger
     global current_backend