Fix prep_resize() stale system_metadata issue

In compute manager, the _prep_resize() method can clobber the
caller's instance['system_metadata'] if it is in dict format,
and leave's the caller's instance object stale after an update.
This seemingly obscure fix isn't hit until the next patch where
we actually have a way to encounter this specific situation.

Related to blueprint compute-api-objects

Change-Id: I4b3a20c483dd632149c7808daf4986c00dc72890
Merge "Fix prep_resize() stale system_metadata issue"