Fix an import error on nova driver
Fix an import error on nova driver

Nova has moved 'arch' to 'fields.Architecture' [1], which caused
the breakage. This commit fixed it.

[1] https://review.openstack.org/#/c/356067/

Change-Id: I0cc7228571746961be5c1597bbb3cfde80bdd519
Closes-Bug: #1646609

kevin-zhaoshuai/zun,kevin-zhaoshuai/zun,kevin-zhaoshuai/zun