Revert "Issue #1798732 by Dean Reilly, westie, alexpott, sun: convert...

Revert "Issue #1798732 by Dean Reilly, westie, alexpott, sun: convert install_task() variable to use state system."

This reverts commit d3edf501.

This caused unexpected errors with installation on sites with existing
settigs.php files. Rolling back until keyvalue store is available during
installation.