Add standard constants for CPU architectures

Introduce a nova/compute/arch.py module to serve a similar role
to nova/compute/vmmode.py, by providing constants for known
CPU architectures and validation/canonicalization APIs

Related-bug: #1348624
Change-Id: Idd4907b5488dd0832c15783d87d57d77ae654519
Merge "Add standard constants for CPU architectures"