Fixes a Ip address already in use exception

Without this patch svc_monitor tries to
allocate the same public ip twice.

Since the following patch introduced
a check a exception is raised is that
case.

https://review.opencontrail.org/#/c/2884/

This patch removes the double
allocation.

Closes-bug: bug/1376741
(cherry picked from commit 46931415bdca93061eaf04ac6060b59c541950b4)
Change-Id: Ia8a50e027847eecfb04dfd399151d36f05d300a1