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.

builder/openstack/access_config.go

@@ -115,10 +115,6 @@ func (c *AccessConfig) Prepare(t *packer.ConfigTemplate) []error {
 		}
 	}
 
-	if c.Region() == "" {
-		errs = append(errs, fmt.Errorf("region must be specified"))
-	}
-
 	if len(errs) > 0 {
 		return errs
 	}