Architecture property updated in snapshot libvirt

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 architecure
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 libvirt driver during a snapshot.

Fixes: bug #1176173
Change-Id: Ic5b4e3d462c08df167e14522fa25a7cf857c6aad
Merge "Architecture property updated in snapshot libvirt"