docker: access system_metadata as a dict

This bit of code causes a problem when moving to objects.  The Instance
object presents system_metadata as you would normally expect (a dict).
The data is in a different format when working with a db instance.  Use
the handy helper utility to normalize it into a dict.

Change-Id: Ieca9ab449a07374ed2c9cdaffdfd54783df7a866
Merge "docker: access system_metadata as a dict"