Improper address:port joining

Similarly to bug 1478943, there's a number of place in Juju that incorrectly join addresses and ports. net.JoinHostAddr should be used instead a naive join using fmt.Sprintf. Not doing this correctly results in invalid address:port pairs when the address is IPv6.

Here are the problem areas:

provider/local/config.go: return fmt.Sprintf("%s:%d", c.bootstrapIPAddress(), c.storagePort())
provider/manual/config.go: return fmt.Sprintf("%s:%d", c.bootstrapHost(), c.storagePort())
provider/manual/config.go: return fmt.Sprintf("%s:%d", c.storageListenIPAddress(), c.storagePort())
state/address.go: newAddrs[i] = fmt.Sprintf("%s:%d", addr, port)
state/backups/backups_linux.go: memberHostPort := fmt.Sprintf("%s:%d", args.PrivateAddress, ssi.StatePort)
state/backups/restore_test.go: c.Assert(dialInfo.Addrs, gc.DeepEquals, []string{fmt.Sprintf("%s:%d", privateAddress, statePort)})
state/backups/restore.go: Addrs: []string{fmt.Sprintf("%s:%d", privateAddr, ssi.StatePort)},
utils/ssh/ssh_gocrypto.go: addr: fmt.Sprintf("%s:%d", host, port),