Fix passing url keyword argument to __send_request

Signed-off-by: Amr Ali <amr@databracket.com>
Parse the response in download_backend_hostkey as JSON

Signed-off-by: Amr Ali <amr@databracket.com>
Parse the API key in both upload-key and start cases

Signed-off-by: Amr Ali <amr@databracket.com>
Assign the API key as the agent's username

Signed-off-by: Amr Ali <amr@databracket.com>