Fixed instance authentication

This pull request simplifies how instance credentials are managed:

If no credentials are explicitly configured, jclouds will create an SSH key pair to access and bootstrap the instances (this can be controlled by the `autoCreateKeyPair` option in the `GoogleComputeEngineTemplateOptions`). If users provide their own access configuration in the template options, jclouds will just use those.

All ComputeService live tests are now passing.

/cc @danbroudy