[RHEL/6] Start using <sub idref=""> variable substitution feature / ability that got introduced in openscap v1.0.8

OpenSCAP 1.0.8 (IIRC) introduced the ability to substitute / expand variable value present in <sub> 
element.

Start using this feature by calling expansion for appropriate variables within XCCDF 
descriptions where this is appropriate instead of using hard-coded values.

Tested on RHEL/6, works as expected.

Please review.

Thanks, Jan.