Cope better with out of sync bm data.

The baremetal hypervisor tracks references to instances which are
deleted asynchronously from the commit to the nova bm. As such, its a
normal but rare condition for a bare metal node to refer to a deleted
(and even garbage collected) nova instance. We should treat such
instances are deleted rather than erroring on any call through
list_instances(), permitting starting new instances and listing
instances to work - fixing bug: 1096722.

Pathologically, the database may suffer permanent skew, which means we
need a way to fix it (and thats tracked as a separate bug).

Change-Id: Ic21ff66b1fc0ad64bb5feff26291873b96d20e4e
Merge "Cope better with out of sync bm data."