Prevent Instance.refresh() from returning a new info cache

If an older node does a refresh() on an instance, conductor will
return a new copy of the InstanceInfoCache, which the client
will balk at. Instead, add a refresh() method to info_cache and
call it from Instance.refresh().

Closes-bug: #1265607
Change-Id: I6a4787a23cd851cf56958c61f8b0559a79847ca0
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 "Prevent Instance.refresh() from returning a new info cache"
Merge "Translate the snapshot_pending state for old instances"