environs/ec2: add public bucket field to configuration.

environs/ec2/config.go

 package ec2
 
 import (
         "fmt"
         "launchpad.net/goamz/aws"
         "launchpad.net/juju/go/environs"
         "launchpad.net/juju/go/schema"
 )
 
 // providerConfig is a placeholder for any config information
@@ skipping 88 matching lines @@
         }
         return &c, nil
 }
 
 func maybeString(x interface{}, dflt string) string {
         if x == nil {
                 return dflt
         }
         return x.(string)
 }