Fix 500 error in SpecialNovaSudoer

Hosts that are in the scheduling, building, or error state are in
the list of instances returned from nova, but don't have ldap dns
entries yet. This special page maps the instances to their dns
name for sudo.

Change-Id: Ibd1fc15a98cd96d4999b2f19d09b1f574a87a1c9