Add support for non-ephemeral storage

Hi,

We have recently switched to the latest plugin version (as soon as #133 got merged), and this became a problem on out OS instance, because we have no support of ephemeral storage (i.e. when no volumes are being created for the VM). Due to that, slaves creation have stopped working on out instance. We've been able to fix it as follows: [these](https://github.com/iamanikeev/openstack-cloud-plugin/commit/ba346801ba5d18b5cc6bb1e8716b52a1e2307b16) [two](https://github.com/iamanikeev/openstack-cloud-plugin/commit/1410c583c306193a7b5285b079038c1c1d116aa6) commits (these are 2 links). The question is: are we alone with this problem? If not, we could make ephemeral storage an option, and let user chose whether to use it or not.