Improving test_volumes_get testcase
Improving test_volumes_get testcase

Volume is defined in the following code. "volume = None" is
unnecessary, delete it. 'id' is uesd above, move it upper.
Add verification of size.

Change-Id: I0557f8b3ebfee2fb946664c7bd190a09ad31aa97

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