Revision of scs-0101-v1-entropy

Created by: mbuechse

Hi everyone.

I tried to apply the structure suggested by scs-0001. Besides, I did a kinda deep dive into the topic of entropy, and I revised the draft accordingly. Please have a look.

Most important insights to me:

• linux does not treat RDRAND/RDSEED like a classical hardware rng (it doesn't appear as /dev/hwrng and it's automatically part of the entropy pool)
• therefore rngd is only required on the host if it does feature a true hw rng
• for the same reason (namely, that linux automatically uses RDRAND), it seems to me that it's mostly up to the user's taste whether they want to use virtio-rng

edit just mention https://github.com/SovereignCloudStack/issues/issues/234 to make the connection