Comment 2 for bug 1529439

Reviewed: https://review.openstack.org/261793
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=a2cfd9fcd67f392d9ad70868fa358afc7aa1b4c3
Submitter: Jenkins
Branch: master

commit a2cfd9fcd67f392d9ad70868fa358afc7aa1b4c3
Author: lzklibj <email address hidden>
Date: Sun Dec 27 14:08:36 2015 +0800

    Unify exceptions for assign router to dvr agent

    validate_agent_router_combination use two different exceptions
    for assigning a router to an agent in 'dvr' mode:
      RouterL3AgentMismatch: assign dvr router to legacy agent.
      DVRL3CannotAssignToDvrAgent: assign dvr router to (another) dvr agent.

    This should be unified to one single exception, for routers on agent in
    'dvr' mode should be only scheduled, not allowed to be manually assigned.

    Change-Id: I3673c4c6852105f86b3aac390d0aabc75944de9d
    Closes-Bug: #1529439