Cinder lvm backends shoudl use targetcli

iscsi targets nowadays are managed by linux kernel and targetcli utility. For many distributions this is a default choice while old tgtd userspace daemon is obsolete and is not found in many general distro repositories (Cent7, RHEL7, Oracle Linux 7). Using targetcli and kernel based iscsi targets will eliminate need for tgtd container.

http://linux-iscsi.org/wiki/Targetcli
http://atodorov.org/blog/2015/04/07/how-to-configure-iscsi-target-on-red-hat-enterprise-linux-7/
https://en.wikipedia.org/wiki/LIO_%28SCSI_target%29