discovery service authentication

[sdpeters]

This depends on the basic NVMe-oF discovery feature (#63). Needed for https://github.com/ceph/ceph-nvmeof/issues/119

We'd like a host to be able to authenticate the discovery service (it is what t says it is, and what the host expected to find), and vice/versa (the discovery service can confirm the host is what it claims to be).

This will be used to reveal Ceph gateway groups to only specific authenticated hosts (not just anyone who asks).