ec2: remove db.instance_system_metadata usage

Update some direct db API calls getting and updating instance system
metadata with usage of the Instance object.  This is the last of
db.instance_* usage in the ec2 API.

Change-Id: I9270ddd2c6a91e85f1c0ed684cb0e440a1587cf2
Merge "ec2: remove db.instance_system_metadata usage"