[digitalocean|compute] power_state_tests fixes

Do not wait for the server to be ready before running the tests since
the server may be powered off.
[digitalocean|compute] list_ssh_keys request mock fixes

Removing ssh_pub_key was making tests fail when mocking randomly.
[digitalocean|compute] include response body in exception when request fails
[digitalocean|compute] Improved tests reliability

Better handling of power state related tests