settings.py Add netboot banner default value

The NETBOOT_SECURITY_BANNER was missing on the settings.py module (it
was only included on the example file). Set it on settings.py as well so
it is used by default.

Author: SantiagoTorres

settings.py

@@ -225,4 +225,7 @@
 
     INSTALLED_APPS = list(INSTALLED_APPS) + ['debug_toolbar']
 
+## Display a warning if serving netboot images on downgraded ciphers
+NETBOOT_SECURITY_BANNER=True
+
 # vim: set ts=4 sw=4 et: