Fix sys_meta access in prep for instance object

Fix various places that access instance['system_metadata'] as if it's a
list of sqlalchemy models.  A helper util method was created in a
previous patch set in nova/utils... so this patch converts things to
use it.

Left one use in _init_instance() in compute/manager.py because it's
being addressed with a different patch set.

Related to blueprint unified-object-model

Change-Id: I876a6c57f64d61d64069c884bf0414c3596e1aa0
Merge "Fix sys_meta access in prep for instance object"