Trigger subscription from vrouter for dns-server when dns-server is down for faster convergence

To ensure faster convergence for dns-server when the connected server goes down, we need the
faster convergence from clients so the dns records can be in sync with fixed-policy.

Change-Id: I839fa28332d36c8f455c3da506c2810817de3ea7
Closes-Bug: 1401221
Merge "Trigger subscription from vrouter for dns-server when dns-server is down for faster convergence" into R2.0