Always start open-iscsi daemon on the compute host

Otherwise nova-compute tries to start it manually
when trying to attach the first volume, and then stumbles
over it because detection hasn't happened yet.
(cherry picked from commit 2a7f9671dfeebf22fbfa42e8d70a3b006041838f)
Ignore return value of open-iscsi start

the open-iscsi start command incorrectly returns "failed"
when there is no active sessions. It is quite normal that
there is no active session initially before the service
started..
(cherry picked from commit 54039c9aaf76f281aac393919cb240c9d3003e8f)