iscsiadm: fix discovery record management

iscsiadm: fix discovery record management

We recently added the ability to use and update discovery
records. when this was done, we had
iscsiadm -m discovery -t st -p ip:port
use the record value if found. This will break compat with
older releases and it is a little strange because
iscsiadm -m discovery -p ip:port [ -o ... | -l ]
works on or uses the discovery record.

So this patch reverts iscsiadm behavior so that
iscsiadm -m discovery -t st -p ip:port
uses the iscsid.conf settings, and adds a new --discover|-D
command that can be used to force discovery.

iscsiadm -m discovery -p ip:port --discover

will force discovery using the record with the ID ip:port.