centralized router is incorrectly scheduled

When agent_mode=dvr only distributed routers should be
scheduled to that agent.  This change fixes this problem
by not allowing centralized routers to be scheduled
to an agent configured with agent_mode=dvr

Closes-bug: 1350667
Change-Id: I34e118535094520f2cee853f39d582a625acde20
Merge "centralized router is incorrectly scheduled"