List ComputeHostNotFound as a client exception

service_get_all_by() in nova.conductor.manager should list
ComputeHostNotFound as a client exception for rpc. This to prevent
the rpc layer from logging a traceback when this exception is raised.
This is an expected type of exception that should be left to the
client to handle.

Resolves bug 1154597.

Change-Id: I656e264526673fa10e47692903a5f30f813b479b