provider/maas bridge script is not idempotent

[Andrew McDermott]

The add-juju-bridge.py script is not idempotent; running the script against an /etc/network/interfaces file that already has bridged interfaces will create another iface stanza and indirectly bridge the existing interface. This will be a problem if the bridge script gets run again as an upgrade step.

For example:

bridgescript_test.go:117:
    s.assertScriptWithPrefix(c, v.expected, v.expected, v.prefix)
bridgescript_test.go:73:
    c.Check(strings.Trim(output, "\n"), gc.Equals, expectedConfig)
... obtained string = "" +
... "auto lo\n" +
... "iface lo inet loopback\n" +
... "\n" +
... "iface eth0 inet manual\n" +
... "\n" +
... "iface test-br-eth0 inet manual\n" +
... "\n" +
... "auto test-br-test-br-eth0\n" +
... "iface test-br-test-br-eth0 inet static\n" +
... " address 1.2.3.4\n" +
... " netmask 255.255.255.0\n" +
... " gateway 4.3.2.1\n" +
... " bridge_ports eth0\n" +
... " bridge_ports test-br-eth0\n" +
... "\n" +
... "auto eth0:1\n" +
... "iface eth0:1 inet static\n" +
... " address 1.2.3.5"
... expected string = "" +
... "auto lo\n" +
... "iface lo inet loopback\n" +
... "\n" +
... "iface eth0 inet manual\n" +
... "\n" +
... "auto test-br-eth0\n" +
... "iface test-br-eth0 inet static\n" +
... " address 1.2.3.4\n" +
... " netmask 255.255.255.0\n" +
... " gateway 4.3.2.1\n" +
... " bridge_ports eth0\n" +
... "\n" +
... "auto eth0:1\n" +
... "iface eth0:1 inet static\n" +
... " address 1.2.3.5"

[Andrew McDermott (frobware)]

WIP branch: https://github.com/frobware/juju/tree/1.25-lp1553915