imgstorage: avoid concurrent execution of tgt-setup-lun

we need this non-sense because tgt-setup-lun can not be run
concurrently (yes you heard this correctly) so need this crap
to make sure only one tgt-setup-lun will run concurrently

Obviously I'm not the only one with this problem:
http://www.spinics.net/lists/linux-stgt/msg03353.html

I didn't want to patch the official Centos RPM but i prefered to
wrap the original script with another bash script