Fix update of display_prefix on parent prefix edit

Modifying the parent prefix (assignment) of a host now updates the
display_prefix correctly. This was simply missed before - perhaps during
refactoring of trigger functions.

Part of #515.
Add test for checking calc of children

Part of #515
Fix calculation of children

Simple off by one bug..

Fixes #515.
Merge branch '515-fix-display_prefix-update'