When GenericExecutionStep instance is created, some general purpose
.spec file variable values are exported to environment:
- RELEASE_STRING
- RELEASE_VERSION
- RELEASE_DESC
This commit improves the support to script hooks, making possible to pass
arguments to them via .spec statements (outer_chroot_script, outer_chroot_script_after,
inner_chroot_script, inner_chroot_script_after, pre_iso_script)