[rfr] Only pass volume_size if it is greater than zero

[jtopjian]

A recent change in Nova now enforces volume_size to be greater than zero. Since volume_size is an optional parameter, it should now only be sent when it has a positive non-zero value.

[jtopjian]

The change in Nova that caused this was implemented in both master and stable/mitaka, so this will affect people once they update to a forthcoming Mitaka point release.

https://github.com/openstack/nova/commit/eb0d70a7c985e39d4f14d3e71d5514412d56e4d7

[coveralls]

Coverage increased (+0.002%) to 81.084% when pulling b7fea37353c44b0e2dd055e2323311e950b84a44 on jtopjian:bfv-volume-size-fix into f28452dea119b3fc68a303e5ac3a4f5fd7e67012 on rackspace:master.

[jrperritt]

LGTM. +2

[jtopjian]

Thank you!