Fix acceptance test (image id not fouund)

It is searching for Image ID 23b564c9-c3e6-49f9-bc68-86c7a9ab5018, which either doesn't exist anymore or at least doesn't existing in the region/account I'm using.

I changed it to find by name, so the test should be less fragile and rackspace specific.

The failing test was:
```sh
go run /Users/Thoughtworker/go/gophercloud/src/github.com/rackspace/gophercloud/acceptance/03-get-server-details.go /Users/Thoughtworker/go/gophercloud/src/github.com/rackspace/gophercloud/acceptance/libargs.go ...
panic: Image 23b564c9-c3e6-49f9-bc68-86c7a9ab5018 (Ubuntu 12.04 LTS) not found.
```