Refactor cluster test to use non-ambiguous cluster URL

[xcoulon]

in https://github.com/fabric8-services/fabric8-auth/blob/master/controller/clusters_blackbox_test.go#L30 we use the default cluster service URL which is http://f8cluster, but if the user's DNS setting include a "search domain" that attempts to resolve the URL, then the test may fail. This was my personal experience with numericable.fr (my local ISP) as a search domain, which would return http://f8cluster.numericable.fr which in turn returns a 403 Forbidden error.
If the test uses an URL such as http://f8cluster.example.com, then the test passes (error is 502 Bad Gateway, as expected).
To fix that, we should provide a custom configuration to the controller that is used in the test.