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
Merge "Avoid auto-scheduling for distributed routers"