Adding boot from volume support for OpenStack.

This change adds support for boot from volume to the OpenStack CPI.  To use, add the following property to the Micro BOSH or BOSH deployment manifest:

    cloud:
      plugin: openstack
      properties:
        openstack:
          boot_from_volume: true

Changes:
* Add a reference to the fog Volume API which supports created volumes with an imageRef
* Create a boot volume before creating the VM with the disk size specified by the flavor
* Add a block device mapping to the VM on creation to mount the new volume at /dev/vda
* Skip setting the "personality" property on VM creation since file injection is not supported with BFV