https://bugzilla.redhat.com/show_bug.cgi?id=1331552

Instead of disabling auto-login of all scsi targets (even those
that do not "belong" to libvirt), use iscsiadm's "--op nonpersistent"
during discovery of iSCSI targets (e.g. "iscsiadm --mode discovery
--type sendtargets") in order to avoid the node database being altered
which led to the need for the "large hammer" approach taken by
commit id '3c12b654'.

This commit removes the virISCSITargetAutologin adjustment (eg. the setting
of node.startup to "manual"). The iscsiadm command has supported this mode
of operation as of commit id 'ad873767' to open-iscsi.