Replace ListServers hack with update ServerById

[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.
Merge pull request #1307 from lflux/better-serverbyid-fix

Replace ListServers hack with update ServerById