[digitalocean|compute] added destroy_ssh_key request and tests
[digitalocean|compute] added get_ssh_key request and tests
[digitalocean|compute] added create_ssh_key request tests
[digitalocean|compute] added SshKey model, collection and related tests
[digitalocean|compute] added ssh related requests, model and collection

Added new requests and model/collection to the compute service

- get_ssh_key
- destroy_ssh_key
- ssh key model/collection