iSCSILogicalUnit don't set the correct sn

Hello,
when I've multiple LUNs, the iSCSILogicalUnit-agent set only for the first LUN the correct sn, all others get a different one, I guess something is wrong in Line 419 or so. It looks like that only the first sn will be set correct, I add some echo's output to the iSCSILogicalUnit script:
```
diff /usr/lib/ocf/resource.d/heartbeat/iSCSILogicalUnit iSCSILogicalUnit.org
49,51d48
< echo "" >> /tmp/iscsilun_debug.log
< echo date >> /tmp/iscsilun_debug.log
<
62d58
< echo ${OCF_RESOURCE_INSTANCE} >> /tmp/iscsilun_debug.log
64,65d59
< echo -n "die sn: " >> /tmp/iscsilun_debug.log
< echo $sn >> /tmp/iscsilun_debug.log
68,69d61
< echo -n "die ocf_sn: " >> /tmp/iscsilun_debug.log
< echo $OCF_RESKEY_scsi_sn >> /tmp/iscsilun_debug.log
417,418d408
< echo -n "vor if reskey_sn: " >> /tmp/iscsilun_debug.log
< echo ${OCF_RESKEY_scsi_sn} >> /tmp/iscsilun_debug.log
421,422d410
< echo -n "in if schleife: " >> /tmp/iscsilun_debug.log
< cat /sys/kernel/config/target/core/iblock_${OCF_RESKEY_lio_iblock}/${OCF_RESOURCE_INSTANCE}/wwn/vpd_unit_serial >> /tmp/iscsilun_debug.log
447d434
< echo "fertig ........................" >> /tmp/iscsilun_debug.log
```
the output is:
```
date
iscsi-lun0
die sn: d0e5396557d77dc7ba590376f1bf5b0f
die ocf_sn: d0e53965
vor if reskey_sn: d0e53965
in if schleife: T10 VPD Unit Serial Number: d0e53965
fertig ........................

date
iscsi-lun1
die sn: db01ac2635d5fd02aa628af69b9542fc
die ocf_sn: db01ac26
vor if reskey_sn: db01ac26
in if schleife: fertig ........................

date
iscsi-lun2
die sn: 0acb0780261a5de6f191831f74a13a80
die ocf_sn: 0acb0780
vor if reskey_sn: 0acb0780
in if schleife: fertig ........................
```
(some other errors here but for quick and dirty its ok)
so I will look deeper into it

Start with the iSCSILogicalUnit script around Line 419 and compare the multiple-LUN debug output with the values from /sys/kernel/config/target/core/iblock_${OCF_RESKEY_lio_iblock}/${OCF_RESOURCE_INSTANCE}/wwn/vpd_unit_serial. Reproduce the case with iscsi-lun0, iscsi-lun1, and iscsi-lun2, then verify that each LUN receives its correct serial number.

Written by the indexing model from the issue text.