[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.
So this basically becomes:

    if (parentId == 0L) {
        RemoteSpace space = confluence.getSpace(spaceName);
        if (space != null) parentId = space.getHomePage();
    }

ideally, space would not be null because that would indicate a different configuration error (spaceName does not exist)