Fix ironic delete fails when flavor deleted

The ironic virt driver looks up the flavor of an instance
when it is going to delete it. This is to obtain extra
specs details that are not available in the instance details.
If the flavor has been deleted this lookup fails and causes
the delete to fail.

The fix makes the lookup include deleted flavors. Note that
extra specs handling is changing in nova, so this code is
likely to become obsolete when they are available by
other means.

Change-Id: I47ba78abfe60e82226acc6a17752db503d9f21d8
Co-Authored-By: Nicholas Randon <nicholas.randon@hp.com>
Co-Authored-By: Phil Day <phil.day@hp.com>
Closes-Bug: #1400269
(cherry picked from commit c4eab7062301b8f3b2de2358c589aee4c53074ef)
Conflicts:
        nova/tests/unit/virt/ironic/test_driver.py
        nova/virt/ironic/driver.py