Create default network in volume tests

Volume negative tests create servers without specifying network,
causing the "Multiple possible networks found" exception
when more than one network exists. Enable default network creation
(create_default_network=True) in these tests to avoid this error

Related-prod: PRODX-51554
Change-Id: I2c836d9e96276b084a1af6d323e4a7775cd2ef13
1 file changed