iscsi.service: do not logout all sessions during shutdown

During shutdown we should logout only from the sessions we've
started with this service. Trying to logout from all sessions
will result in a hung system in case of root on iSCSI.

References: bnc#880338

Signed-off-by: Hannes Reinecke <hare@suse.de>
iscsid.service: Fixup dependencies

The iscsid service does not depend on any target service, and
doesn't have to wait for any network device to come online.
And we can use 'Type=simple' to remove one layer of indirection
and get rid of the PID file.

References: bnc#880338

Signed-off-by: Hannes Reinecke <hare@suse.de>
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