Additional headless environment variables

Added support for configuration of headless behavior (reuse of a running
xvfb display, the display port, and whether or not to kill xvfb after
script execution) through additional environment variables.

Removed previously used at_exit hook as it was redundant (headless
provides this behavior through the :destroy_at_exit argument.

Bug: 71602
Change-Id: Ie17e3d011f9974b39a9140d90bad199c54408c68