LBaaS: handle NotFound exceptions in update_status callback

LBaaS agent may send update_status requests to server on objects
which were already deleted from db: this is due to creating and
deleting objects with a high rate (like tempest API tests do).
As a result errors and stacktraces appear in server and agent logs.
The proposed solution is to catch NotFound exceptions and print a warning.

Change-Id: I4446b678893d0bda578ad7ccdf3f109cb1c91b4d
Closes-Bug: #1260682
Merge "LBaaS: handle NotFound exceptions in update_status callback"