Fix the bug that running a test twice on the same host results in the second test stopping the containers and network of the first test:
* Have the subnet name in the network name, so there's no collision of network names when cleaning up the network.
* Don't clean up the network when creating a new network. Rely on the clean up trap to clean up the network instead.
* Don't set NET_NAME when sourcing jenkins-common.sh anymore, as network_create will set it. There's also network_bridge_create, which only gets used by osmo-ran (which we don't run in jenkins). Add a check to this function to ensure NET_NAME is set before calling it, which osmo-ran is already doing.
