Unify exceptions for assign router to dvr agent

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