Catch InstanceInfoCacheNotFound during build_instances

instance.refresh() is raising InstanceInfoCacheNotFound which is logged
by the conductor and would cause all following builds to fail in a
multiple build request situation.  The cause appears to be a race
condition which should be addressed separately, but for now this
exception should be caught, logged, then ignored.

Change-Id: Ibf9e6687933ee90dad54b1998b243ef7256d79ca
Closes-bug: 1324793
Merge "Catch InstanceInfoCacheNotFound during build_instances"