overcloud: cinder-tgt cinder-lio via EXTRA_ARGS

Updates devtest_overcloud.sh so that we set the configured
cinder-lio and/or cinder-tgt elements via the
OVERCLOUD_CONTROL_DIB_EXTRA_ARGS variable.

Also, updates the default and docs in devtest_variables.sh.

This change makes it possible to easily use cinder-lio as
a default. This is the prefered default on Fedora and RHEL 7
systems.

Change-Id: Ic7059e2e37a443d871e563c4e6d6ca1515bf7c63
Merge "overcloud: cinder-tgt cinder-lio via EXTRA_ARGS"