race condition in network noop driver

There's a race condition in the network noop driver that prevents the multivip tests (https://review.opendev.org/c/openstack/octavia-tempest-plugin/+/664462) from being run concurrently.

There are some tricks in the noop driver for associating subnets with networks.
Basically, a call to get_subnet(subnet_id) returns a subnet that belongs to the network (network_id) that was used in the previous call to get_network(network_id) :

def get_network(self, network_id)
    sets 'current_network' to network_id

https://opendev.org/openstack/octavia/src/commit/d7bba6c5af3c0f6d99ea8063b54ec3e895a5eea4/octavia/network/drivers/noop_driver/driver.py#L217

def get_subnet(self, subnet_id)
    associates subnet_id to the 'current_network'

https://opendev.org/openstack/octavia/src/commit/d7bba6c5af3c0f6d99ea8063b54ec3e895a5eea4/octavia/network/drivers/noop_driver/driver.py#L229

it works fine when using tempest with one thread.
But the behavior is undefined when using multiple threads, two threads may call get_network at the same time, then any calls to get_subnet return a subnet that may be associated to the network of another test.