[google|tests] Fix disks and servers live tests

I've noticed that `rake live[google]` gives a lot of errors, so I've decided to fix it. For now I have fixed issues with disks and servers tests. 

Mainly the issues was connected with adding asynchronous requests in `destroy` methods, in this way resources from one test were not removed before execution of another test. I've solved it with generating randomized names for resources.

If someone needs, [here](https://gist.github.com/allomov/464642096193dada7cad) is my script to remove all test resources generated by this tests and not removed because of tests failure. 

@icco do you think it is needed to implement live tests ? as sometimes they are far away from development version.

@geemus I needed to edit `model_helper.rb` (see the [link](https://github.com/allomov/fog/blob/google-live-tests/tests/helpers/model_helper.rb#L12)) to [wait resource](https://github.com/allomov/fog/blob/2f6dfc9ceb0aff528623c30601945ec3889c0854/tests/google/models/compute/disk_tests.rb#L8) is ready before remove it. 
@allomov why not use `model.wait_for { ready? }` here?