Generate an "ephemeral" static key pair when using Noise

[ralexstokes]

Refer: https://github.com/libp2p/py-libp2p/blob/master/libp2p/security/noise/transport.py#L28-L33

If noise_privkey there is None, then this module should generate a random key for further use.

Right now we just end up with an exception trying to do cryptography over None.

[ralexstokes]

future ref: https://github.com/libp2p/specs/tree/master/noise