Changes

network_create: put SUBNET in NET_NAME

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.

Related: OS#5802
Change-Id: Ifcd384272c56d585e220e2588f2186dc110902ed