Update create_volume.rb

support for availability_zone when creating volumes. 
according to http://docs.openstack.org/api/openstack-block-storage/2.0/content/Create_Volume.html
and http://docs.openstack.org/api/openstack-compute/2/content/POST_os-volume-v2_createVolume__v1.1__tenant_id__os-volumes__volume_id__ext-os-volume.html