Initial lbinfo in server metadata

This puts the load balancers a server needs to be added to into the server metadata when it's created.

The data will look like:

    {
        "rax:auto_scaling_group_id": "11111-11111....",
        "rax:auto_scaling_lbids": "[1234, 4567]",
        "rax:auto_scaling:lb:1234": "{\"port\": 80}",
        "rax:auto_scaling:lb:4567": "{\"port\": 81}"
    }

If the metadata has changed, a message is also logged - not sure if this is useful but might be good for us to see if this ever happens.
This is really a consequence of not having nonlocal (or perhaps more generally of not having dynamic binding? :)), but since check_metadata only makes sense after wait_for_server has fired, it parses easier in my brain if the functions read that way as well (i.e. the function definition is after wait_for_server's one); even though of course all that matters is the .addCallback order :)