Medium: iscsi: run iscsi discovery only when necessary

Some platforms (most?) use records the open-iscsi database in
/etc/iscsi/nodes to store some configuration data, such as
whether to login to the target on boot (node.startup). This
database is rebuilt on iscsi discovery or discoverydb (depending
on the version). Better avoid that operation unless necessary.
This patch makes the RA run discovery only if the record for the
given target and portal is not present.