Enable the user to enforce validity of the gateway IP

Fixes bug 1096532

A new configuration variable is added to enable the user to indicate
if the gateway should be validated on the subnet. For backward
compatibility this is set as False by default.

Change-Id: Ieadd60a945d34703bfee7576aa3b2ff7da3143d4
Merge "Enable the user to enforce validity of the gateway IP" into stable/folsom
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