DVR routers should not be auto scheduled according to router_auto_schedule flag

[Armando Migliaccio]

DVR routers have specific rules regarding when and where the placement of the namespaces (SNAT and IR namespaces) occur, therefore auto-placing based routers should be avoided as it may conflict with these.

We should also document inline what these rules are.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/115754

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/115754
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=1105d732b2cb6ec66d042c85968d47fe6d733f5f
Submitter: Jenkins
Branch: master

commit 1105d732b2cb6ec66d042c85968d47fe6d733f5f
Author: armando-migliaccio <email address hidden>
Date: Wed Aug 20 12:15:42 2014 -0700

    Avoid auto-scheduling for distributed routers

    The reason for this is twofold:

    - It may relieve contention on DB access while
      both servers and l3 agents are busy setting up
      and syncing routers down respectively.

    - It prevents accidental placement of namespaces
      during the L3 sync_routers process, as auto
      scheduling without taking into account the state
      of the L3 agents, as well as the state of the
      routers being processed, may overrule the placement
      decision made during router operations.

    Partial-bug: #1356121
    Partial-bug: #1359326

    Change-Id: Ia677ce212145d6cee65adeb1d8ae594e6ac5e34d