fence_scsi: fix registration handling if ISID conflicts
ISID (Initiator Session ID) belonging to I_T Nexus changes for
RHEL based on the session ID. This means that the connection to
the device can be set up with different ISID on reconnects.

fence_scsi treats same key as a tip to ignore issuing registration
to the device but if the device was registered using a different
ISID, the key would be the same but the I_T Nexus (new ISID) would
not have access to the device.

Fixing this by preempting the old key and replacing with the current
one.