Fix orphaned instance from get_by_uuid() and _from_db_object()

We need to set _context on the Instance object we create during
get_by_uuid(), but more specifically during _from_db_object().
In order to properly initialize things like nested objects that
themselves need a _context, we should pass it to _from_db_object()
always.

Same thing for InstanceInfoCache.

Related to blueprint unified-object-model

Change-Id: I09ab6b5d7d8961b69294536885e8432943b3a611
Merge "Fix orphaned instance from get_by_uuid() and _from_db_object()"