Replace ListServers hack with update ServerById

Earlier hack to iterate over ListServers is unnecessary after 
[gophercloud#168](https://github.com/rackspace/gophercloud/issues/168) has been
patched to not panic on non-existing server id's. If an error is returned,
check if the error is a 404 first before bailing.

This will need a `make updatedeps` to update gophercloud.