Adding volume_type configuration for Micro BOSH

Adding the ability to set a new "persistent_type" property in the Micro BOSH manifest in order to specify the volume type for the persistent disk.  This gets passed to the create_disk function in the CPI in the same way that the new disk_pool functionality uses, via the cloud_properties parameter.

Micro BOSH manifest would look as follows:

    resources:
      persistent_disk: 20480
      persistent_type: gp2
      cloud_properties:
        instance_type: flavor
        availability_zone: az1

If the new property is not specified than no value is passed to the create_disk function as before.

Aaron Huber
Intel Corporation