Avoid error on double-delete of nested stack

When deleting a stack that has a nested stack previously deleted, the
process fails because the nested stack cannot be retrieved from the
database.
To solve the bug, the nested stack is now retrieved from the database
ignoring the deleted stacks before the deletion process begins. Then
a NotFound exception is raised, and then when it's catched the nested
stack will be ignored.

Change-Id: Ib863527dbfb0200b833a4e90b5a4fd50f469c1ce
Closes-Bug: #1261436
Merge "Avoid error on double-delete of nested stack"