Catch NotFound for keystone delete operations

If heat-engine is terminated mid-delete, a stack could become
undeletable because of NotFound exceptions being raised on keystone
delete stack domain operations.

This change wraps all keystone delete calls and ignores any NotFound
exceptions raised.

Change-Id: I032b04b1391590a6f9493b65bddff331b07722f0
Closes-Bug: #1308812
Merge "Catch NotFound for keystone delete operations" into stable/icehouse