Fix error that instance could not be found

The instance may have been deleted when calling 'force_delete_server'.
So this patch ignores exception 'lib_exc.NotFound' that avoid
error in tearing down stage.

Change-Id: I29d6d1a41e3fb1bbbdb1f16ef4e54916df2e1ec7
Closes-Bug: #1896865
1 file changed