Bug Fix - Boto fails to construct AWS API parameters correclty

When using boto to create a Launch Configuration, sometimes you need to pass
in a value for the 'block_device_mappings' parameter by passing in a list of
Block Device Mappings as described in the documentation for
boto.ec2.autoscale.launchconfig.LaunchConfiguration. The cause of the bug that
this commit attempts to fix occurs when Boto fails to construct
the correct request for the Amazon API. When attempting to use boto to create
a Launch Configuration with a list of Block Device Mappings, the Amazon would
return a '400 Bad Request' and claim

<ErrorResponse xmlns="http://autoscaling.amazonaws.com/doc/2011-01-01/">
  <Error>
    <Type>Sender</Type>
    <Code>MalformedInput</Code>
    <Message>Top level element may not be treated as a list</Message>
  </Error>
  <RequestId>XXXXXX-XXXX-XXXX-XXXX-XXXXXXX</RequestId>
</ErrorResponse>

In short, the Amazon API requires there to be a '.member' in the key of the
key/value pair of the request parameters having to do with Block Device
Mappings. For instance, the POST parameter before this bug fix was:

BlockDeviceMapping.1.DeviceName=%2Fdev%2Fsdd

The correct value (obtained from running the commandline developer tools and
showing the request) should be

BlockDeviceMappings.member.1.DeviceName=%2Fdev%2Fsdd

Notice the '.member' after 'BlockDeviceMappings'. As I am not intimately
familiar with boto, I hope this does not break anything. I also do not
know if this is the right fix for this bug. Maybe the
create_launch_configuration() method should be calling
AutoScaleConnection.build_list_params() instead of
BlockDeviceMapping.build_list_params()?