Update the cinder-lio element to use the venv dir

This updates the cinder-lio element to make use of the
CINDER_VENV_DIR variable which is set by the cinder element.

This allows source installs of this element to work correctly
when using common-env or a custom Cinder venv directory.

Change-Id: Idb8a97180045869595416cf0b17aec70a63d9055
Merge "Update the cinder-lio element to use the venv dir"