env_process: Add restore_image postprocessing parameter

Now when setting restore_image = yes, the postprocessing
code will *always* restore the image from backup. With
this we can implement an option in the test runner to
always restore the image from backup between tests.

Useful for folks that want to be extra careful, and
ensure the guest image is always pristine and clean
on every new test.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>