Remove redundant architecture property update in powervm snapshot

During an instance snapshot if the architecture property is defined
in the image it is updated twice in the snapshot image.
The snapshot is created with the metadata values from the
"instance_system_metadata" table and then, if defined, the architecture
property is updated from the value in the base image in that moment.

There is no reason to treat this property differently from others.
Also, it didn't respect the "non_inheritable_image_properties" option.

This change removes the special behavior for the architecture property
in the powervm driver during a snapshot.

Fixes bug 1189969

Change-Id: I05ebcaef7b75387827917b5b223868794d98e0aa
Merge "Remove redundant architecture property update in powervm snapshot"