DVS: provide a dhcp_driver class for plugin

[OpenStack Infra]

https://review.openstack.org/349248
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commit 709cc0c154ae3522033ad2bf79518c71aa9aabc5
Author: Gary Kotton <email address hidden>
Date: Sun Jul 31 02:17:31 2016 -0700

    DVS: provide a dhcp_driver class for plugin

    Enable the plugin to override the base DHCP DeviceManager.

    This patch does the following:
    1. provide a class that overrides the default. This is required
       so that we can override the device_manager class
    2. Introduces two new configuration variables:
        - dhcp_override_mac - enables the driver to override the
          neutron port mac address. This should be used when the DHCP
          agent is running on a VM that is connected to the DVS. This
          can save the admin the bother of setting 'forged transmits'
          to enable. This should be the MAC address of the VM
        - dvs_integration_bridge - the name of the bridge to plug the
          interface. This may be a dedicated bride that is connected
          to the DVS

    This enables one to use the simple DVS plugin and actaully get a
    DHCP address.

    NOTE: a port group that can see all of the tags should be
    configured. This is the port group that the dvs_integration_bridge
    should be attached to.

    DocImpact

    Change-Id: I6b0742b363e551e09a8c44fd59c698a90d180efd
    Depends-on: cee3664a77487750c2c5f68f1df97f3d80362e17