Remove unnecessary parameters on create_volume API

Current Tempest client passes a null value to Nova's create_volume
API if tests don't specify description_name and metadata attributes.
However, that is wrong behavior because these attributes are optional
and Tempest doesn't need to specify them instead of specifying null
values. This behaivor will be a problem because Nova v2.1 API doesn't
allow this kind of values by strict input validation.

This patch makes create_volume API client skip passing null values
in this case.

Change-Id: I16249afeb254dfce1c4b155e2e3746500f6a9ea5