[change] Get rid of paramiko workaround if paramiko allows to pass preferred_rsa_keys

[nemesifier]

Follow up of #643 and #644.

If the feature requested in paramiko to pass preferred_rsa_keys (https://github.com/paramiko/paramiko/issues/2054) is accepted and implemented, we can get rid of the hack introduced in https://github.com/openwisp/openwisp-controller/commit/062800f28ebf03f6c77c4b4228e8bb0ee3dd00ae, replacing it with passing preferred_rsa_keys which give priority to sha1 for backward compatibility.