disk-usage pluging: separate configuration and history serialization into different files

To be able to store only the configuration using scm-sync-configuration plugin, it may be good to separate the plugin configuration and data storage into different files.
At the time, the disk usage history is saved together with the configuration. This causes a periodical update of the configuration file which was synced to the scm. (see also JENKINS-18187)

To fit scm-sync-configuration naming conventions, the history may be named "hudson.plugins.disk_usage.DiskUsageProjectActionFactory.history". (*.xml was the configuration file indication for scm-sync-configuration in jenkins base directory)