OpenStack builder: Make region not required

Perhaps "region" is required when using a public provider like
Rackspace? It's not required for my private cloud from Metacloud. I
suspect a lot of private clouds have only a single region and thus don't
need "region" to be specified.
Make region required for rackspace but not other providers

Update test to make it pass.
Merge pull request #1418 from msabramo/openstack_builder_region_not_mandatory

builder/openstack: Make region not required if not rackspace