Fix #269: Managed Area layer legend items not appearing.

As a result of fixing #255, where we removed parent node ids from the
list of visible layer ids, the map legend for nested layers fails to
render on the map export.

This change will append all parentLayerIds from visible layers to the
PrintTask template when the map is exported.
Merge pull request #272 from kdeloach/topic/issue-269

Fix #269: Managed Area layer legend items not appearing.