Ignore InstanceNotFound while getting console output

During we cold-migrating or resizing an instance to another host,
or during deleting it, the get_console_output method may raise an
InstanceNotFound excetpion if the instance is not on the hypervisor,
this is an expected error, so we should add it to expected
exceptions list in compute manager.

Closes-bug: #1292339
Change-Id: I23a99438d73826f01ff5662130dcbfa72dc39a6e
Merge "Ignore InstanceNotFound while getting console output"