[FIXED JENKINS-15472] Added parentId property

Added a new job-configuration field for parent page ID#.
Plugin verifies that page ID exists and returns an error message if not.
It works with the `parentId` set to `0L`, but it creates a page that is outside the space hierarchy.  (That is, it doesn't appear in the sidebar or anything).

However, that was my experience with the plugin before I started making any changes to it -- the page wouldn't appear under the Space home.

Is it worth it to have the plugin set the parent to the Space home page if a parent is not provided?