Code review comment for lp:~themue/juju-core/go-ec2-security-group-config

This is going in the right direction, but this problem is not specific
to EC2, so we can take another step forward and have it ready for the
next providers. I suggest introducing:

     firewall-mode: default / global

With an internal API such as:

     func (c *Config) FirewallMode() FirewallMode { ... }

     type FirewallMode string

     const (
             FwDefault FirewallMode = "default"
             FwGlobal FirewallMode = "global"
     )

https://codereview.appspot.com/6596051/