Do not log WARN messages about lack of L3 agents for DVR routers

DVR routers are scheduled only when there is need to: for instance when
there's an external gateway associated with it, or when there are
interfaces attached to subnets on which VM's are actually being deployed.

Therefore, do not log misleading messages that apply only to the
centralized case.

Closes-bug: 1358998

Change-Id: Ica7f0d25fe576f05a9adf8eb8239704f09c0b0ac
Merge "Do not log WARN messages about lack of L3 agents for DVR routers"