Fix LVM iSCSI driver tgtadm CHAP authentication

Currently CHAP Authentication in LVM iSCSI driver with tgtadm does not work.
This is because the tgtadm helper creates the target configuration file
with an 'IncomingUser' entry, which is ignored by tgtd.
This patch fixes it to 'incominguser'.

Change-Id: I14871985a2a916834122f849238f05b75726bc1a
Closes-Bug: #1329214
(cherry picked from commit e3563891545c801726d227f752cf99488ed5c7dd)