[aws|compute] Updates 'connection' references

`connection` is being deprecated within Fog::Model because it actually
refers to the Fog::Service not the Fog::Connection.

This replaces references to the instance variable to use the accessor so
will continue to work following deprecation.

lib/fog/aws/models/compute/image.rb

@@ -31,7 +31,7 @@ module Fog
 
           if(delete_snapshot && root_device_type == "ebs")
             block_device = block_device_mapping.detect {|block_device| block_device['deviceName'] == root_device_name}
-            @connection.snapshots.new(:id => block_device['snapshotId']).destroy
+            connection.snapshots.new(:id => block_device['snapshotId']).destroy
           else
             true
           end