Remove a redundent variable
Remove a redundent variable

Instead of creating a new variable for volume_id,
we can simply do self.volume['id']

Change-Id: I5b077f403e22ce1ff6833bc397afd21bf4d3e498

Tesora/tesora-tempest,openstack/tempest,masayukig/tempest,vedujoshi/tempest,cisco-openstack/tempest,sebrandon1/tempest,Tesora/tesora-tempest,masayukig/tempest,Juniper/tempest,Juniper/tempest,sebrandon1/tempest,openstack/tempest,vedujoshi/tempest,cisco-openstack/tempest