Use default_notification_level when notification

Fix bug 1089773

Notifications in quantum api do not honor default_notification_level
in quantum.conf and always use INFO level. On the other hand
dhcp-agent refers to default_notification_level.
If default_notification_level is set to a value other than INFO,
dhcp-agent cannot receive notification from quantum server.

Change-Id: Ie3ae576d62e91651aa59b2324ec114716181107f