Translate the snapshot_pending state for old instances

Icehouse introduced an image_snapshot_pending state which Havana
compute nodes do not understand. This adds a translation for that
state when we are handling save() for a havana-era Instance object.

Closes-bug: #1265618
Change-Id: I36028defc3591cc6e31c7ba763a5c7f076832179
Merge "Translate the snapshot_pending state for old instances"