Check if interface exists in bridge prior to adding.

Check if interface exists in bridge prior to adding.

This fixes bug 1019730. The fix for bug 1000406 ensures that return
values of shell commands are checked. The command
    utils.execute(['brctl', 'addif', bridge_name, interface],
                   root_helper=self.root_helper)
would cause an exception if there was more than one attachment
on the network. The reason for this was the interface already existed
on the bridge.

Change-Id: I8a1f6cc7be930c04ce302d7f87814b9bd5bed129