issue #432: Display unique values for iSCSI targets

When creating a iSCSI pool, the iSCSI targets were listed twice on
openSUSE systems as returned by iscsiadm.

opensuse-vm:~ # iscsiadm -m discovery -t sendtargets  -p localhost
[::1]:3260,1 iqn.2014-09.mydomain:alinefm
[fe80::5054:ff:fe29:5551]:3260,1 iqn.2014-09.mydomain:alinefm

To avoid confusion, display unique values.

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>