Configure Cinder to use lioadm on CentOS/RHEL 8

In CentOS/RHEL 8 there is no scsi-target-utils package, nor is it
available in EPEL. It is removed from kolla in [1]. In RHEL 7 and beyond
the LIO kernel subsystem can be used instead of the tgtd daemon.

This change removes support for the SCSI target daemon on CentOS/RHEL 8.
The 'tgtd' image is no longer available for CentOS/RHEL 8.

[1] https://review.openstack.org/#/c/613815/5

Change-Id: I718fc16cde2dd177b2a1c2f79b932426034897fe
Related: blueprint centos-rhel-8