Nova review updates for _node_resource

In review https://review.openstack.org/#/c/111425/14 it has been
raised that if the cpu_arch is not set, we should be using Nova
helper functions instead of 'NotFound'.  Also we should be
canonicalizing the user provided config value.

Once this is approved and merged, it will be proposed back into
the Nova tree.

Change-Id: Idfe0730daa22c6550a1a15b45f481635d2eb5195
Merge "Nova review updates for _node_resource"