In order to implement the backup and restore of log stores, that
are created as subplugins of the tool_log plugin , we need to
extend subplugins support from activities to virtually any plugin.

Basically that implies moving the add_subplugin_structure() method from
its current, restricted, activity level to general restore_structure_step.

This commit implements the change in restore, covered with tests verifying
old, bc behavior and also new, general one.