Port update with existing ip_address only causes exception

Fixes bug 1132852

Change-Id: Iedb0dce3ece270eb0a58af6aa8800e2047dc920d
Add scheduling feature basing on agent management extension

3rd part of blueprint quantum-scheduler

1. Allow networks to be hosted by certain dhcp agents.
Network to dhcp agent is a
many to many relationship. Provide a simple
scheduler to schedule a network randomly
to an active dhcp agent when a network or port is created.
2. Allow admin user to (de)schedule network to a
certain dhcp agent manually.
3. Allow routers to be hosted by a certain l3 agent.
Router to l3 agent is a many to one relationship.
Provide a simple scheduler to
schedule a router to l3 agent if the router is not
scheduled when the router is  updated.
4. Auto schedule networks and routers to agents when agents
start.
5. Only support ovs plugin at this point

Change-Id: Iddec3ea9d4c0fe2d51a59f7db47145722fc5a1cd
Merge "Port update with existing ip_address only causes exception"
Merge "Add scheduling feature basing on agent management extension"