Create default network in volume tests

Volume negative and filter 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
2 files changed