Added AgentSchedulerDbMixin to Midonet Plugin

AgentSchedulerDbMixin implements create_or_update_agent that is needed for
the rpc communication between the dhcp agent and quantum server

Change-Id: If61d1b2adf60cd97d923cd9dfa2d5ab8af9c57e3
Fix: #24
Signed-off-by: Rossella Sblendido <rossella@midokura.com>