Fix dhcp agent scheduler to be resistant to race coditions

[Eugene Nikanorov]

DHCP ChanceScheduler has _schedule_bind_network method which can detect duplicate entry.

Need to add the same ability to auto_schedule_networks.

That might be important for the case when api server and rpc_server run in different processes.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/100903

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/100903
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=3867174bc82b7fd85dd79bc0cc5625a15df2d8fb
Submitter: Jenkins
Branch: master

commit 3867174bc82b7fd85dd79bc0cc5625a15df2d8fb
Author: Eugene Nikanorov <email address hidden>
Date: Wed Jun 18 17:02:42 2014 +0400

    Fix auto_schedule_networks to resist DBDuplicateEntry

    This exception may happen if API and RPC workers are in different
    processes.
    Also make minor refactoring of auto_schedule_networks method
    to avoid unnecessary db queries.
    Add missing unit tests and adjust unit test naming style

    Change-Id: I6460744e2cffec0b9f009da071597374d8c027f6
    Closes-Bug: #1331456