1.20 client can't destroy a 1.21 or 1.22 environment

Error is related to the translation from localhost loopback proxy urls to bridge ip...

In other words, the call getAddressForInterface(networkBridge) returned empty string

The same thing happens when attempt to destroy a 1.21 environment with a 1.20 client.

I've updated this ticket to indicate this.

the failure happens in cmd client before it goes over the wire. Specifically at

environ, err := environs.NewFromName(c.envName, store)

this will eventuate in the call to env config which will try to substitute network-bridge adresses which in these versions is ""...
Hence, it falls over before reaching server.

This a representative stack trace...

....../src/github.com/juju/juju/provider/local/config.go:74 (0x80965e)
        com/juju/juju/provider/local.(*environConfig).networkBridge: debug.PrintStack()
....../src/github.com/juju/juju/provider/local/environ.go:301 (0x80c87a)
        com/juju/juju/provider/local.(*localEnviron).resolveBridgeAddress: networkBridge := config.networkBridge()
....../src/github.com/juju/juju/provider/local/environ.go:286 (0x80c64a)
        com/juju/juju/provider/local.(*localEnviron).SetConfig: if err := env.resolveBridgeAddress(cfg); err != nil {
....../src/github.com/juju/juju/provider/local/environprovider.go:80 (0x80fa1c)
        com/juju/juju/provider/local.environProvider.Open: if err := environ.SetConfig(cfg); err != nil {
....../src/github.com/juju/juju/provider/local/config.go:1 (0x815da2)
        com/juju/juju/provider/local.(*environProvider).Open: // Copyright 2013 Canonical Ltd.
....../src/github.com/juju/juju/environs/open.go:164 (0x4d0e5e)
        com/juju/juju/environs.New: return p.Open(config)
....../src/github.com/juju/juju/environs/open.go:127 (0x4d0c30)
        com/juju/juju/environs.NewFromName: env, err := New(cfg)
....../src/github.com/juju/juju/cmd/juju/destroyenvironment.go:78 (0x40b585)
        (*DestroyEnvironmentCommand).Run: environ, err := environs.NewFromName(c.envName, store)
....../src/github.com/juju/cmd/supercommand.go:321 (0x46fc9a)
        com/juju/cmd.(*SuperCommand).Run: err := c.subcmd.Run(ctx)
....../src/github.com/juju/cmd/cmd.go:247 (0x46b193)
        com/juju/cmd.Main: if err := c.Run(ctx); err != nil {
....../src/github.com/juju/juju/cmd/juju/main.go:77 (0x4105ea)
        Main: os.Exit(cmd.Main(jcmd, ctx, args[1:]))
....../src/github.com/juju/juju/cmd/juju/main.go:174 (0x412a04)
        main: Main(os.Args)
/usr/lib/go/src/pkg/runtime/proc.c:220 (0x44471f)
        main: main·main();
/usr/lib/go/src/pkg/runtime/proc.c:1394 (0x446c50)
        goexit: runtime·goexit(void)

Older clients can't destroy newer environments because a) default network bridge name changed to empty string and b) reasonable default for network bridge name was only provided via getter.

This change moves the guess of reasonable default to config creation.

We may not need change 1.21 because the network change is specific to 1.22.

Comment #2 says this issue was verified on a 1.21 environment so on that basis 1.21 will need to be fixed.

In addition to unit tests for envirnonment setup, I have also manually tested - both 1.22 and 1.21 env were successfully destroyed by 1.20 client.