OpenStack "openstack_volume_size" not included in json results output

OpenStack provider specific flags such as "openstack_volume_size" are not making their way into the result json which could be misleading, since defaults make their way in.

Flag Values:

```
2016-09-27 13:46:35,551 browbeat MainThread INFO Flag values:
--openstack_volume_size=5
--machine_type=m1.small
--os_type=rhel
--benchmarks=copy_throughput
--openstack_network=browbeat_private
--image=centos7
--cloud=OpenStack
--timing_measurements=runtimes
--copy_benchmark_mode=cp
--run_uri=browbeat
--openstack_floating_ip_pool=browbeat_public
--openstack_image_username=centos
```

And in the newline delimited results json:

```
{"labels": "|data_disk_0_type:standard|,|zone:nova|,|scratch_disk_type:standard|,|perfkitbenchmarker_version:v1.7.0|,|num_striped_disks:1|,|machine_type:m1.small|,|scratch_disk_size:500|,|data_disk_0_size:500|,|data_disk_0_num_stripes:1|,|image:centos7|,|cloud:OpenStack|,|vm_count:1|", "timestamp": 1474998635.938512, "metric": "cp throughput", "official": false, "value": 197.3035439137134, "test": "copy_throughput", "run_uri": "browbeat-eed78df5-5389-4b37-b717-ed062cea8d99", "owner": "stack", "product_name": "PerfKitBenchmarker", "unit": "MB/sec", "sample_uri": "3f3a66a2-7a1c-4be7-9378-1a9533518d92"}
```

Looking through the results, you might think there is a 500G disk vs just a 5G cinder volume. Shouldn't "openstack_volume_size" be included under "labels"?