Catch NotFound error during loadbalancer deleting

There is a situation, when  we request resource data
from db during loadbalancer resource deleting. If
there is no such data, NotFound exception raised and
stack delete failed with this error. To prevent this
behaviour, there is a need to catch NotFound exception
in delete method of loadbalancer.

Change-Id: Ie52bc3059e91bd25effc53bbd255bce6948509ac
Closes-bug: #1381700