Default to ed25519 host key type
The change is backwards compatible - if /nso/etc/ssh/ssh_host_rsa_key
exists then keep using it but emit a warning. New installations default
to ed25519.
The new default also makes it easier to enable LSA because newer
versions of NSO will refuse to connect to a host using rsa key type
without explicitly allowing it.
Closes #274 (closed)