Check system_metadata type in _populate_instance_for_create

The _populate_instance_for_create method is passed a dictionary
of base instance values during create but it wasn't checking to
see if system_metadata was a dict or not which can lead to a
TypeError when booting an instance via the cells scheduler.

Fixes bug 1195549

Change-Id: I60b853eda48726b3bb752b7e3542b0fca74bc926
Merge "Check system_metadata type in _populate_instance_for_create"