Add FLUSH operation

  * Add a FLUSH operation that will:
    1) Drop all items from the midonet_tasks table
    2) add all items in the neutron databases as CREATE operations
       in the midonet_tasks table

Change-Id: I8d59550935027993f10f6710ee6093e4795983d7
Signed-off-by: Joe Mills <joe@midokura.com>